I'm assuming you have a PCI Express slot. I'm not expert, but the card I see discussed frequently as a good "bang for the buck" is the Nvidia GeForce GTX 260. However, I do remember reading something recently about an ATI card that "changed the game" so to speak, by being a comparable card at a lower price.
I just clicked over and checked, though, and even the 260 is over $200.
I would just check out this article at Tom's Hardware on the best graphics cards for the money at various price points:
www.tomshardware.com/reviews/radeon-geforce-graphics,2086.html