Probably the 7800GT would be the best bet.
Originally Posted by laboitenoire
There's also the 6800GS, which while not as good as the 7800GT, will last you a while. If you wait a bit, you can get the 7800GS, which is coming to everyone sometime soon. Although from the sound of it, the 6600GT is the best card, as I take you are running on an AGP board.