The main reason why I bought X1800 XL over 7800 GT back then, was because I knew I'd play all my games with 16x Anisotropic-Filtering, and that I never really cared about jagged egdes, so no need for those fency "transparent" and "super" A-A modes and what not. During the first week when R520's first reviews were released, where they compared pictures with A-F from nVidia and HQ-AF from ATi then I told myself there would be no reasons I'd go with nVidia as long as image quality goes.
The only thing I really like about nVidia's current GPU architecture is their "Ultra Shadow" technology, which does make a lot of difference in processing power needed to calculate a scene with complex shadowing effects, such as Doom 3, Prey and Quake IV. It also helped in pretty much all games I knew about, using Stencil Shadows, such as Warhammer 40,000 and Age of Empires III.
And then I realized that I was tight on money, and that I'd have to go with the XL version of the X1800 series, where I wouldn't have the opportunity to use a lot of shadows in games, and usually just turn it Off, or decrease its quality via in-game options in the end. So I went with ATi based on such decisions, and also because I heard it was an over-clocking beast, at least back then, I still remember. And they were right, because right now it runs very smooth at OC'ed frequencies, never had a single issues due to OC'ing.
This time around (G80 Vs R600) I'll wait for reviews, lots of them, will read them carefully, then eventually make my decision, where as usual my budget will have the last word.