As stated, both cards are nearly equal in performance. They trade blows in different games - 8800 GTS 512 gets the with the edge at the lower resolutions, and the GTX is better a higher resolutions/high IQ levels. As you can see in the game averages below, both cards are nearly identical in performance with a tiny advantage to the GTX.
"Our today’s tests show that this new graphics card from the $299-349 price category is indeed as fast as the Nvidia GeForce 8800 GTX, which comes from the $549-649 sector, in most of games despite having less memory and lower memory bandwidth. The only exception is the resolution of 1920x1200 where the influence of the memory subsystem parameters on performance is substantial especially with enabled FSAA"
A 24" monitor runs @ 1920x1200... well mine does anyhow