Look at the specs and you'll see the 6800 is the better card
6600GT
500/1000 128bit memory
8 pipes
6800
350/600 256bit memory
12 pipes
The clocks on the 6800 are about twice as effective as the 6600GT clocks, so you can double them and find it's a faster card, and it has more pipes as well. For only a bit more you can much better performance, and you can overclock and try soft modding to 6800GT.