The fact is its only a difference when you get past the high end cards.
When you buy a $400 card from nVidia, your getting awesome performance in most games, and great performance in games like HL2 and other ATI sided engines.
When you pay the same for an ATI... you get performance a little better than nVidia on ATI based games and you get decent performance on other games.
Those are comparable to eachother, of course it is different when you get lower in price.
Aroudn the $300 mark the x800 XL holds the podium, $200 - 6600gt.
But when you spend so much money on a card get the one that is UNIVERSALY better for the price.
nVidia rules when you compare the price to the performance.
When you think about dropping 600-800 bucks on ONE ati card, be smart and get TWO 6800gts for the SAME price, with INSANE performance!!!
Granted you have to be building the complete computer, ebcause you have to have an SLI MB but still, ATI just plain isn't cost effective.
PS - Whoever said that thing about the old 8500 64mb...I have one too and it does more then I'd expect a 64mb card to do!