Here is the one of the key reasons why the R600 marchitecture falls behind the G80 GTX version. While Geforce 8800 GTX or G80GTX has 32 Texture Memory Units (TMUs) Radeon HD 2900 XT has 16 only.
TMUs are responsible for getting the textures in your games and the more you have, the faster you can handle the texture work.
Still, the Radeon HD 2900 XT at 750 MHz core is about 30 percent faster in raw clock speed from Geforce 8800 GTX at 575 MHz and finishes the texture cycle some 30 percent faster than Nvidia.
In spite of that this is not really an ideal solution as Nvidia has twice as many texture memory units. The only hope is that ATI has more effective TMUs, but this is one big marchitectural disadvantage over the G80 marchitecture.