No comptetition..the 8600 GTS is not in the same league.
The weak spot with the 8800GS and cards in the same class, is its 384MB of 192-bit memory. On lower resolutions this isn't a problem, however if you crank up the detail and resolution you'll see a large drop in frames.
On that basis the choice for me would be between a 9600GT and faster 8800GT. Both offer 512MB of 256-bit memory.http://forums.anandtech.com/messageview.aspx?catid=31&threadid=2172005
Lowest NewEgg prices after rebate:
8800 GS = $99
512MB 9600 GT = $130
512MB 8800 GT = $134
Latest GPU Comparisons http://www.tomshardware.com/reviews/graphics-card,1923.html