8800GS is $100 after MIR now. It's nearly as fast as a 9600GT.
I'd guess the 9600 GT is a lot faster than the 8800 GS.
Here's the memory bandwidth (a good indicator of performance) on the various 9600 GT's from the EVGA site:
62.4 GB/s-60.8 GB/s-57.6 GB/s-57.6 GB/s
Here's the memory bandwidth for the various 8800 GS's:
45.6 GB/s-38.4 GB/s
Possibly why the 9600 is more expensive?