As long as you don't play on ridiculously huge resolutions (1900*1200 or bigger) I think they would perform about the same, but the 8800GT 256mb will still have the upperhand. On lower resolutions you will notice the difference between the 8800GT 256mb version and 9600GT... The 8800GT 256mb will still beat the 9600GT, but why not spend 20 dollars for a pretty big performance difference?
The 8800GT beats the 8800GS, and the 9600GT should be about the same performance as the 8800GS (only the one beats the other were the other loses to the other again on a different place, they're good in different things).