Making most from partitioned tables in SQL Server 2005
SQL Server 2005 table partitioning provides many improvements in terms of
manageability and availability. This is an enhancement on taking the performance advantage on the aspects of manageability by allowing improvements to metadata only switch-in and switch-out of a
partition of data. Regardless of whether you are using partitioning, the selection of a parallel
plan is determined by the number of CPUs, query cost, available memory and
current workload. The remaining considerations outlined here are valid only if
all of these allow using parallel plan.
More on the subject is available from SQL BOL and from 2 SQL Server MVPs: Kimberley Whitepaper & Nigel's article.