XFX 9600 GT 512MB Review » Page 6 - Testing: Crysis - Overclockers Club
That review should give you a pretty clear indication between the 8800GT and 9600GT. Its not huge, (especially when overclocked) but its there. Also, don't forget, you can very well overclock the 8800GT too
PS The performance of both of them has improved a bit in Crysis with the newer driver releases.