The 9600GT is not an 8800GT....it has half the stream processors and lower clocks. It performs surprisingly well though, and no-one seems to know why. But the 512mb 8800GT beats it in benchmarks.
Basically what I think happened was Nvidia were preparing the 9xxx series, and saw ATi were about to launch some decent cards....the 3800 series. Panicking, Nvidia released their 9-series cards as 8-series cards, so that they had something out there to compete right away. And now we're seeing the 8800GTS 512 core being re-used in the new 9800 series.
The 8800 Ultra and the 9800GTX are pretty much neck and neck at lower resolutions, but as you increase the pixels, the more
RAM and the better memory interface help pull the Ultra ahead.