In my experience people abuse tempdb, please consider:

select [Month] = DATENAME(mm,SomeDateTime),

--[mm] =  datepart(mm, SomeDateTime),

Amount  = SUM(SomeAmount)

from

(

select   TOP (10000000)

SomeDateTime = RAND(CHECKSUM(NEWID()))*7305 + CAST('2005' AS DATETIME),

SomeAmount   = CAST(RAND(CHECKSUM(NEWID()))*100 AS MONEY)

FROM master.sys.all_columns ac1

CROSS...