Enough slander, ATi's high range cards being outperformed by a 6600GT in Doom 3 is utter bollocks, and all of you know that. The 6600GT does reasonably well, considering its price versus the ATi cards, but don't stretch the truth.
is a very comprehensive guide of most videos you'll find, anywhere.
Pixel Shader 3.0 is also pretty irrelevant seeing as how you'd be hardpressed to find a game which utilizes it. I haven't seen any review ever bring PS 3.0 as an argument as to why nVidia is superior to ATi.
Even then, I have yet to see any benchmarks which utilize this shader. It seems you are just assuming that nVidia cards will perform fantastic in this enviroment, but my question is, what if they don't?
To the oirginal poster, don't make any purchases until you've done your research. Make sure you know what you're getting and don't base your purchase on any previous experience with either company. Performance/price edge is always shifting.
If you insist on buying an ATi, I will recommend the x800XL.