George is pretty much dead on the money, but a comment about multiple partitions in the primary.

The only time I would recommend using partitions without multiple file groups is for partition swapping.

Not that partitioning can't still help you, particularly if you can alter your queries to make sure that most calls stay in a particular division of the partition. It is, however, mostly just an administrative tool at that point rather than an optimizing one, as you could get equivalent performance from the partition optimization (within reason) with just a more effective indexing mechanic.