I will have to disagree with you. A 8800gt/9800gt is pretty close to a 9800/8800gtx and will surpass it when overclocked. The gtx does have a higher bit-rate and a few more stream processors, so it will not slow down as much at higher resolutions. However, if AA is not factored in there wont be much of a FPS difference between the two.
The 9800gtx and 250 gts are very similiar, but the 250gts averages ~10-15fps better then the 9800gtx
btw, I played crysis on a 8800gt back in the day at 1920x1200. I think my avg fps was around 24 at high settings.