No GTX exists for 6800 series. In terms of performance, it goes like this 6800 Ultra > 6800GT > 6800GS > 6800XT > 6800 vanilla.
7600GT has 128 bit memory interface vs 256 bit memory bus of 6800 ultra. But it matches and even exceeds performance with better ALU's 'n stuff. (I don't know the exact details.)