My information comes from the July edition of PC Pro, they did a comparison on a wide range of cards from the 6200 right up to the ATI X1900 XTX, now I've reread the article, it is clear that the Ati X1900 XTX and X1900 XT, and the X1800 XT are faster then the 7900 GTX, the X1800 XT is 5 frames faster, then X1900 XT is 7 frames faster and the X1900 XTX is 11 frames faster.
However this ONLY applies to one game, Call of Duty 2, and it's not unusual for games to be sponsered by companies like ATI and nVidia to optermise the engines for one or that other. I suspect that's what's happened here, given that most other games are very close in results, the differences being 1 - 3 FPS even between the 7900 GTX and X1900 XTX. At the end of the day, both chipsets for the majority of games are very closely matched, so basically look at the prices rather than the speed. At this level we are talking about £250 - £350, and unless you are mega rich you are better off going for something cheaper, and getting a few games and the same time.
The reason I like nVidia is their unified driver system, you go online, you select "GEFORCE and TNT2" then the OS you are using, and lo and behold you have the lastest driver. With ATI it's much less clear cut.
I've also found all their cards (I've owned the TNT2, Geforce 2, Geforce FX5200 and FX5900, and finally the GT6600, and they've all been stable, easy cards to use. When I find a brand that constantly provided stable hardware I stick with it, and while I admit that the last ATI card I used was the Rage Pro, it did have a few problems which is why I went to nVidia in the first place.
