Well haven't even looked at best buy website but your have a very nice gaming system not realy anything to upgrade if you want to blow your 100 bucks on something only I would suggest is maybe another 1gig of memory.
Oh just noticed you didn't put state whether you had a DVD burner that would be good upgrade or a bigger HD
Only thing I can see inside that budget is another gig of RAM. Won't speed games up too much today (only a couple games show any improvement today with 2gig vs 2gig) but newer games need more memory... Besides, then you'll be able to run in dual channel mode which should give you a boost.