A) What's your budget?
B) What's your interface? (AGP, PCI, PCI-E)
A good overall card right now for the average gamer is the nVidia GeForce 7600GT, if you've got PCI-E (though I think they make an AGP version). Good bang for the buck.
I own the second one and I highly recommend it.. Great card, and it doesn't cost too much. I play Oblivion at max settings with no problem, and FEAR at high but not quite max settings with 40 FPS.