I would highly recomend the Nvidia 7000 series. The ideal card for the best performance and price is the Geforce 7600 GT. I have previously owned a XFX GeForce 7600 GT XXX Edition and absolutley loved it. It runs off a 350w. (which is low) and it is 256MB. And for only $120, and dont know who wouldn't.
Only reason why i sold it was because my friend got a 7900GTX and was stupid enough to put it on a 300w. power supply and it fryed his computer trying to start it. So he just gave it to me, and i gave him the 7600