Here is the script that I used in my Dr. DMV presentation this afternoon at PASS. It is a set of pretty heavily commented queries that are very useful for detecting and diagnosing many common performance issues with SQL Server 2008.

-- SQL Server 2008 Diagnostic Information Queries
-- Glenn Berry 
-- November 2009
-- http://glennberrysqlperformance.spaces.live.com/
-- Twitter: GlennAlanBerry
-- SQL and OS Version information for current instance
SELECT @@VERSION AS [SQL Version Info];
--   2008 RTM Builds                    2008 SP1 Builds
-- Build       Description        Build        Description
-- 1600        Gold RTM
-- 1763        RTM CU1
-- 1779        RTM CU2
-- 1787        RTM CU3    -->      2531        SP1 RTM
-- 1798        RTM CU4    -->      2710        SP1 CU1
-- 1806        RTM CU5    -->      2714        SP1 CU2 
-- 1812        RTM CU6    -->      2723        SP1 CU3
-- 1818        RTM CU7    -->      2734        SP1 CU4
 
-- Hardware information from SQL Server 2008 
-- (Cannot distinguish between HT and multi-core)
SELECT cpu_count AS [Logical CPU Count], 
hyperthread_ratio AS [Hyperthread Ratio],
cpu_count/hyperthread_ratio AS [Physical CPU Count], 
physical_memory_in_bytes/1048576 AS [Physical Memory (MB)], 
sqlserver_start_time
FROM sys.dm_os_sys_info;
-- Get sp_configure values for instance
EXEC sp_configure 'Show Advanced Options', 1;
GO
RECONFIGURE;
GO
EXEC sp_configure;
-- Focus on
-- backup compression default
-- clr enabled
-- lightweight pooling (should be zero)
-- max degree of parallelism
-- max server memory (MB)
-- optimize for ad hoc workloads (should be 1)
-- priority boost (should be zero)
-- File Names and Paths for TempDB and all user databases in instance 
SELECT [dbid], fileid, [filename] 
FROM sys.sysaltfiles
WHERE [dbid] > 4 AND [dbid] <> 32767
OR [dbid] = 2;
-- Things to look at:
-- Are data files and log files on different drives?
-- Is everything on C: drive?
-- Is TempDB on dedicated drives?
-- Are there multiple data files?
-- Recovery model, log reuse wait description, 
-- and compatibility level for all databases on instance
SELECT [name], recovery_model_desc, log_reuse_wait_desc, 
       [compatibility_level] 
FROM sys.databases;
-- Things to look at
-- How many databases are on the instance?
-- What recovery models are they using?
-- What is the log reuse wait description?
-- What compatibility level are they on?
-- Clear Wait Stats 
-- DBCC SQLPERF('sys.dm_os_wait_stats', CLEAR);
-- Isolate top waits for server instance since last restart or statistics clear
WITH Waits AS
(SELECT wait_type, wait_time_ms / 1000. AS wait_time_s,
    100. * wait_time_ms / SUM(wait_time_ms) OVER() AS pct,
    ROW_NUMBER() OVER(ORDER BY wait_time_ms DESC) AS rn
 FROM sys.dm_os_wait_stats
 WHERE wait_type NOT IN('SLEEP_TASK', 'BROKER_TASK_STOP', 
  'SQLTRACE_BUFFER_FLUSH', 'CLR_AUTO_EVENT', 'CLR_MANUAL_EVENT',
  'LAZYWRITER_SLEEP')) -- filter out some irrelevant waits
SELECT W1.wait_type, 
  CAST(W1.wait_time_s AS DECIMAL(12, 2)) AS wait_time_s,
  CAST(W1.pct AS DECIMAL(12, 2)) AS pct,
  CAST(SUM(W2.pct) AS DECIMAL(12, 2)) AS running_pct
FROM Waits AS W1
INNER JOIN Waits AS W2
ON W2.rn <= W1.rn
GROUP BY W1.rn, W1.wait_type, W1.wait_time_s, W1.pct
HAVING SUM(W2.pct) - W1.pct < 95; -- percentage threshold for waits
-- Common Significant Wait types with BOL explanations
-- *** Network Related Waits ***
-- ASYNC_NETWORK_IO        Occurs on network writes when the task is blocked behind the network
-- *** Locking Waits ***
-- LCK_M_IX               Occurs when a task is waiting to acquire an Intent Exclusive (IX) lock
-- LCK_M_IU               Occurs when a task is waiting to acquire an Intent Update (IU) lock
-- LCK_M_S                Occurs when a task is waiting to acquire a Shared lock
-- *** I/O Related Waits ***
-- ASYNC_IO_COMPLETION  Occurs when a task is waiting for I/Os to finish
-- IO_COMPLETION        Occurs while waiting for I/O operations to complete. 
--                      This wait type generally represents non-data page I/Os. Data page I/O completion waits appear 
--                      as PAGEIOLATCH_* waits
-- PAGEIOLATCH_SH       Occurs when a task is waiting on a latch for a buffer that is in an I/O request. 
--                      The latch request is in Shared mode. Long waits may indicate problems with the disk subsystem.
-- PAGEIOLATCH_EX       Occurs when a task is waiting on a latch for a buffer that is in an I/O request. 
--                      The latch request is in Exclusive mode. Long waits may indicate problems with the disk subsystem.
-- WRITELOG             Occurs while waiting for a log flush to complete. 
--                      Common operations that cause log flushes are checkpoints and transaction commits.
-- PAGELATCH_EX         Occurs when a task is waiting on a latch for a buffer that is not in an I/O request. 
--                      The latch request is in Exclusive mode.
-- BACKUPIO             Occurs when a backup task is waiting for data, or is waiting for a buffer in which to store data
-- *** CPU Related Waits ***
-- SOS_SCHEDULER_YIELD  Occurs when a task voluntarily yields the scheduler for other tasks to execute. 
--                      During this wait the task is waiting for its quantum to be renewed.
-- THREADPOOL           Occurs when a task is waiting for a worker to run on. 
--                      This can indicate that the maximum worker setting is too low, or that batch executions are taking 
--                      unusually long, thus reducing the number of workers available to satisfy other batches.
-- CX_PACKET            Occurs when trying to synchronize the query processor exchange iterator 
--                      You may consider lowering the degree of parallelism if contention on this wait type becomes a problem
-- Signal Waits for instance
SELECT CAST(100.0 * SUM(signal_wait_time_ms) / SUM (wait_time_ms) 
       AS NUMERIC(20,2)) AS [%signal (cpu) waits],
       CAST(100.0 * SUM(wait_time_ms - signal_wait_time_ms) / SUM (wait_time_ms) 
       AS NUMERIC(20,2)) AS [%resource waits]
FROM sys.dm_os_wait_stats;
-- Signal Waits above 10-15% is usually a good sign of CPU pressure
-- Get CPU Utilization History for last 30 minutes
DECLARE @ts_now bigint = (SELECT cpu_ticks/(cpu_ticks/ms_ticks)FROM sys.dm_os_sys_info); 
SELECT TOP(30) SQLProcessUtilization AS [SQL Server Process CPU Utilization], 
               SystemIdle AS [System Idle Process], 
               100 - SystemIdle - SQLProcessUtilization AS [Other Process CPU Utilization], 
               DATEADD(ms, -1 * (@ts_now - [timestamp]), GETDATE()) AS [Event Time] 
FROM ( 
      SELECT record.value('(./Record/@id)[1]', 'int') AS record_id, 
            record.value('(./Record/SchedulerMonitorEvent/SystemHealth/SystemIdle)[1]', 'int') 
            AS [SystemIdle], 
            record.value('(./Record/SchedulerMonitorEvent/SystemHealth/ProcessUtilization)[1]', 
            'int') 
            AS [SQLProcessUtilization], [timestamp] 
      FROM ( 
            SELECT [timestamp], CONVERT(xml, record) AS [record] 
            FROM sys.dm_os_ring_buffers 
            WHERE ring_buffer_type = N'RING_BUFFER_SCHEDULER_MONITOR' 
            AND record LIKE '%<SystemHealth>%') AS x 
      ) AS y 
ORDER BY record_id DESC;
-- Page Life Expectancy (PLE) value for default instance
SELECT cntr_value AS [Page Life Expectancy]
FROM sys.dm_os_performance_counters
WHERE OBJECT_NAME = 'SQLServer:Buffer Manager' -- Modify this if you have named instances
AND counter_name = 'Page life expectancy';
-- PLE is a good measurement of memory pressure
-- Higher PLE is better. Below 300 is generally bad.
-- Watch the trend, not the absolute value
-- Buffer Pool Usage for instance
SELECT TOP(20) [type], SUM(single_pages_kb) AS [SPA Mem, Kb] 
FROM sys.dm_os_memory_clerks 
GROUP BY [type]  
ORDER BY SUM(single_pages_kb) DESC;
-- CACHESTORE_SQLCP  SQL Plans         - These are cached SQL statements or batches that aren't in 
--                                     stored procedures, functions and triggers
-- CACHESTORE_OBJCP  Object Plans      - These are compiled plans for stored procedures, 
--                                     functions and triggers
-- CACHESTORE_PHDR   Algebrizer Trees  - An algebrizer tree is the parsed SQL text that 
--                                     resolves the table and column names
-- Switch to a user database
USE AdventureWorks;
GO
-- Individual File Sizes and space available for current database
SELECT name AS [File Name] , physical_name AS [Physical Name], size/128 AS [Total Size in MB],
size/128.0 - CAST(FILEPROPERTY(name, 'SpaceUsed') AS int)/128.0 AS [Available Space In MB]
FROM sys.database_files;
-- Look at how large and how full the files are and where they are located
-- Make sure the transaction log is not full!!
-- Top Cached SPs By Execution Count (SQL 2008)
SELECT TOP(25) p.name AS [SP Name], qs.execution_count,
ISNULL(qs.execution_count/DATEDIFF(Second, qs.cached_time, GETDATE()), 0) AS [Calls/Second],
qs.total_worker_time/qs.execution_count AS [AvgWorkerTime], qs.total_worker_time AS [TotalWorkerTime],  
qs.total_elapsed_time, qs.total_elapsed_time/qs.execution_count AS [avg_elapsed_time], 
qs.last_elapsed_time,qs.cached_time
FROM sys.procedures AS p
INNER JOIN sys.dm_exec_procedure_stats AS qs
ON p.[object_id] = qs.[object_id]
ORDER BY qs.execution_count DESC;
-- Top Cached SPs By Total Worker time (SQL 2008). Worker time relates to CPU cost
SELECT TOP(25) p.name AS [SP Name],qs.total_worker_time AS [TotalWorkerTime], 
qs.total_worker_time/qs.execution_count AS [AvgWorkerTime], 
qs.execution_count, ISNULL(qs.execution_count/DATEDIFF(Second, qs.cached_time, 
GETDATE()), 0) AS [Calls/Second], qs.total_elapsed_time, 
qs.total_elapsed_time/qs.execution_count AS [avg_elapsed_time], qs.last_elapsed_time, qs.cached_time
FROM sys.procedures AS p
INNER JOIN sys.dm_exec_procedure_stats AS qs
ON p.[object_id] = qs.[object_id]
ORDER BY qs.total_worker_time DESC;
-- Top Cached SPs By Total Logical Reads (SQL 2008). Logical reads relate to memory pressure
SELECT TOP(25) p.name AS [SP Name],qs.total_logical_reads AS [TotalLogicalReads], 
qs.total_logical_reads/qs.execution_count AS [AvgLogicalReads],
ISNULL(qs.execution_count/DATEDIFF(Second, qs.cached_time, GETDATE()), 0) AS [Calls/Second],
qs.total_worker_time AS [TotalWorkerTime], qs.total_worker_time/qs.execution_count AS [AvgWorkerTime], 
qs.execution_count, qs.total_elapsed_time, qs.total_elapsed_time/qs.execution_count AS [avg_elapsed_time], qs.last_elapsed_time,
qs.cached_time
FROM sys.procedures AS p
INNER JOIN sys.dm_exec_procedure_stats AS qs
ON p.[object_id] = qs.[object_id]
ORDER BY qs.total_logical_reads DESC;
-- Top Cached SPs By Total Physical Reads (SQL 2008). Physical reads relate to disk I/O pressure
SELECT TOP(25) p.name AS [SP Name],qs.total_physical_reads AS [TotalPhysicalReads], 
qs.total_physical_reads/qs.execution_count AS [AvgPhysicalReads],
ISNULL(qs.execution_count/DATEDIFF(Second, qs.cached_time, GETDATE()), 0) AS [Calls/Second],
qs.total_logical_reads AS [TotalLogicalReads], qs.total_logical_reads/qs.execution_count AS [AvgLogicalReads],
qs.total_worker_time AS [TotalWorkerTime], qs.total_worker_time/qs.execution_count AS [AvgWorkerTime], 
qs.execution_count, 
qs.total_elapsed_time, qs.total_elapsed_time/qs.execution_count AS [avg_elapsed_time], qs.last_elapsed_time,
qs.cached_time 
FROM sys.procedures AS p
INNER JOIN sys.dm_exec_procedure_stats AS qs
ON p.[object_id] = qs.[object_id]
ORDER BY qs.total_physical_reads DESC;
    
    
-- Top Cached SPs By Total Logical Writes (SQL 2008). Logical writes relate to both memory and disk I/O pressure 
SELECT TOP(25) p.name AS [SP Name], qs.total_logical_writes AS [TotalLogicalWrites], 
qs.total_logical_writes/qs.execution_count AS [AvgLogicalWrites],
ISNULL(qs.execution_count/DATEDIFF(Second, qs.cached_time, GETDATE()), 0) AS [Calls/Second],
qs.total_logical_reads AS [TotalLogicalReads], qs.total_logical_reads/qs.execution_count AS [AvgLogicalReads],
qs.total_worker_time AS [TotalWorkerTime], qs.total_worker_time/qs.execution_count AS [AvgWorkerTime], 
qs.execution_count, 
qs.total_elapsed_time, qs.total_elapsed_time/qs.execution_count AS [avg_elapsed_time], qs.last_elapsed_time,
qs.cached_time
FROM sys.procedures AS p
INNER JOIN sys.dm_exec_procedure_stats AS qs
ON p.[object_id] = qs.[object_id]
ORDER BY qs.total_logical_writes DESC;
-- Possible Bad Indexes (writes > reads)
SELECT OBJECT_NAME(s.[object_id]) AS [Table Name], i.name AS [Index Name], i.index_id,
        user_updates AS [Total Writes], user_seeks + user_scans + user_lookups AS [Total Reads],
        user_updates - (user_seeks + user_scans + user_lookups) AS [Difference]
FROM sys.dm_db_index_usage_stats AS s WITH (NOLOCK)
INNER JOIN sys.indexes AS i WITH (NOLOCK)
ON s.[object_id] = i.[object_id]
AND i.index_id = s.index_id
WHERE OBJECTPROPERTY(s.[object_id],'IsUserTable') = 1
AND s.database_id = DB_ID()
AND user_updates > (user_seeks + user_scans + user_lookups)
AND i.index_id > 1
ORDER BY [Difference] DESC, [Total Writes] DESC, [Total Reads] ASC;
-- Missing Indexes for entire instance by Index Advantage
SELECT user_seeks * avg_total_user_cost * (avg_user_impact * 0.01) AS [index_advantage], 
migs.last_user_seek, mid.[statement] AS [Database.Schema.Table],
mid.equality_columns, mid.inequality_columns, mid.included_columns,
migs.unique_compiles, migs.user_seeks, migs.avg_total_user_cost, migs.avg_user_impact
FROM sys.dm_db_missing_index_group_stats AS migs WITH (NOLOCK)
INNER JOIN sys.dm_db_missing_index_groups AS mig WITH (NOLOCK)
ON migs.group_handle = mig.index_group_handle
INNER JOIN sys.dm_db_missing_index_details AS mid WITH (NOLOCK)
ON mig.index_handle = mid.index_handle
ORDER BY index_advantage DESC;
-- Look at last user seek time, number of user seeks to help determine source and importance
-- SQL Server is overly eager to add included columns, so beware
-- Get Table names, row counts, and data compression status (SQL 2008 Only)
SELECT OBJECT_NAME(object_id) AS [Table Name], SUM(Rows) AS [Row Count], 
       data_compression_desc AS [Compression]
FROM sys.partitions 
WHERE index_id < 2 --ignore the partitions from the non-clustered index if any
AND OBJECT_NAME(object_id) NOT LIKE 'sys%'
AND OBJECT_NAME(object_id) NOT LIKE 'queue_%' 
AND OBJECT_NAME(object_id) NOT LIKE 'filestream_tombstone%' 
GROUP BY object_id, data_compression_desc
ORDER BY SUM(Rows) DESC;
-- Which tables are largest, which maybe candidates for data compression?
-- Breaks down buffers used by current database by object (table, index) in the buffer cache
SELECT OBJECT_NAME(p.[object_id]) AS [ObjectName], p.[object_id], 
p.index_id, COUNT(*)/128 AS [buffer size(MB)],  COUNT(*) AS [buffer_count] 
FROM sys.allocation_units AS a
INNER JOIN sys.dm_os_buffer_descriptors AS b
ON a.allocation_unit_id = b.allocation_unit_id
INNER JOIN sys.partitions AS p
ON a.container_id = p.hobt_id
WHERE b.database_id = DB_ID()
AND p.[object_id] > 100
GROUP BY p.[object_id], p.index_id
ORDER BY buffer_count DESC;
-- Tells you what tables and indexes are using the most memory in the buffer cache
-- Detect blocking (run multiple times)
SELECT t1.resource_type AS [lock type],DB_NAME(resource_database_id) AS [database],
t1.resource_associated_entity_id AS [blk object],t1.request_mode AS [lock req],                                                                          --- lock requested
t1.request_session_id AS [waiter sid], t2.wait_duration_ms AS [wait time], -- spid of waiter  
(SELECT [text] FROM sys.dm_exec_requests AS r                              -- get sql for waiter
CROSS APPLY sys.dm_exec_sql_text(r.[sql_handle]) 
WHERE r.session_id = t1.request_session_id) AS [waiter_batch],
(SELECT SUBSTRING(qt.[text],r.statement_start_offset/2, 
    (CASE WHEN r.statement_end_offset = -1 
    THEN LEN(CONVERT(nvarchar(max), qt.[text])) * 2 
    ELSE r.statement_end_offset END - r.statement_start_offset)/2) 
FROM sys.dm_exec_requests AS r
CROSS APPLY sys.dm_exec_sql_text(r.[sql_handle]) AS qt
WHERE r.session_id = t1.request_session_id) AS [waiter_stmt],    -- statement blocked
t2.blocking_session_id AS [blocker sid],                         -- spid of blocker
(SELECT [text] FROM sys.sysprocesses AS p                        -- get sql for blocker
CROSS APPLY sys.dm_exec_sql_text(p.[sql_handle]) 
WHERE p.spid = t2.blocking_session_id) AS [blocker_stmt]
FROM sys.dm_tran_locks AS t1 
INNER JOIN sys.dm_os_waiting_tasks AS t2
ON t1.lock_owner_address = t2.resource_address;

Here is the second script, with some suggested settings for an an OLTP workload. As always, YMMV, and you should do your own testing and use your own best judgment depending on your own specific workload.

-- Selected sp_configure settings
-- Glenn Berry
-- November 2009
-- http://glennberrysqlperformance.spaces.live.com/
-- Twitter: GlennAlanBerry
-- Turn on advanced options
EXEC sp_configure 'Show Advanced Options', 1;
GO
RECONFIGURE;
GO
-- See what the current values are
EXEC sp_configure;
-- Set MAXDOP = 1 for the server (For OLTP workloads)
EXEC sp_configure 'max degree of parallelism', 1;
GO
RECONFIGURE;
GO
-- Set max server memory = 3500MB for the server (This depends on several factors)
EXEC sp_configure 'max server memory (MB)', 3500;
GO
RECONFIGURE;
GO
/* These settings are for x64, on a dedicated database server, 
   only running the DB engine. 
Physical RAM         MaxServerMem Setting 
2GB                  1500 
4GB                  3200 
6GB                  4800 
8GB                  6400 
12GB                 10000 
16GB                 13500 
24GB                 21500 
32GB                 29000 
48GB                 44000 
64GB                 60000
72GB                 68000
96GB                 92000
128GB                124000 
*/-- Enable optimize for ad hoc workloads (new for SQL 2008)
EXEC sp_configure 'optimize for ad hoc workloads', 1;
GO
RECONFIGURE;
GO
-- Set default backup compression on (new for SQL 2008, EE only)
EXEC sp_configure 'backup compression default', 1;
GO
RECONFIGURE;
GO
-- Enable CLR (Only if you need it)
EXEC sp_configure 'clr enabled', 1;
GO
RECONFIGURE;
GO
-- See what the current values are
EXEC sp_configure;