A 7600GT has about 24GB/s memory bandwidth, where as a 6800GS even has around 32GB/s memory bandwidth. Performance wise, the 6800Ultra would probably stomp the crap out of the 7600GT.
However, the 7600GT uses less power, has a smaller PCB, and doesnt require any additional power to be supplied to it. Pipeline wise Im unsure of, if I recall correctly, the 7600GT has 12, and I'm fairly certain the 6800GS has 12, but the most a 6800Ultra could have, I'm thinking is 16, but again, I'm unsure of this. I do know however that in past benchmarks the 6800Ultra card often outperformed the 7800GTs.
[EDIT]
JUst like to add, that I own a eVGA 7600GT KO, and in Oblivion is plays @ 1440x900 high settings, and hovers in the 60FPS area. During heavey combat, around 35FPS, and plays very smooth.