Well, nothing good that could be plugged into your existing hardware.
Pretty much build a new entry level rig from scratch.
A Phenom II X4 or Core I5 would be a good starting point. A 1.5GB GTX480 can be had for about $200 new with warranty. Ram is dirt cheap, 8gb of DDR3 1600mhz would be plenty for your needs. http://secure.newegg.com/WishList/PublicWishDetail.aspx?WishListNumber=15598469
What kind of budget do you have to work with? What type of games do you like to play, resolution, and settings?