Have a read through chapter 3: http://www.simple-talk.com/books/sql-books/troubleshooting-sql-server-a-guide-for-the-accidental-dba/
Unfortunately there isn't an answer that always works everywhere. I like to leave maxdop at 0, raise cost threshold (I'll test queries and see the aprox cost where parallelism brings a benefit) then handle exceptions with either query hints, plan guides or resource governor settings.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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