if you've got a PCI x 16 (PCI express 16 slot) then you can run most any PCI card.

Just go to newegg.com and search for cards, and just look at the specs page. It should show

Interface
Interface PCI Express x16

I went with this card, for my 300$ cheapy comp.
http://www.newegg.com/Product/Produc...82E16814136025

If you want to spend 200$ and get a real good card, get something like this. (8800GT 512 MB)
http://www.newegg.com/Product/Produc...GeForce+8800GT