The 8600 ultra wil run 500 core, 1400 memory, and will have 64 shaders and 512mb ram.
With a 256 bit memory bus.
The 8800gts is running at 500 core 1600 memory, and has 96 shaders,320 mb ram. with a 320 bit memory bus. I think the shaders will be clocked higher also. (just a guess)
I would say the gts will outperform the ultra. Faster memory,32 more shaders (clocked higher?),and a faster memory bus.
The ultra seems on par with a 7900gtx. Nvidia mid range new products are usually around the speed of there old flag ship.