Actually, ATI hasn't caught up to NVidia's 6800 Ultra as of yet.. maybe the x900 will match the 6800 Ultra when it becomes available, maqybe even surpass it.. but the 6800 Ultra has held the #1 spot for the last 3 to 4 months now in performance and graphical quality...
I like both cards myself.. NVidia is my favorite, but I do use Radeon from time to time such as currently...
Anyhow both companies keep each other in check and push each other for creating the best of the best when it comes to graphics and gaming.
Also, just for the casual user.. there is basically no visual difference that the human eye can see between the x850 and the 6800 Ultra.. both cards take advantage of all the advancements and shaders in DirectX 9 and all the latest in Open GL.. However NVidia's card has a few things that it can out render and enhance that Radeon can't, but you would be very hard pressed to point it out under any application.. whether your professionsl or casual in the card usage..
To a gamer, the difference is about 1/1000 of a sec difference in framerate rendering.. which in todays standards in gaming is about 10% in performance.. like I stated.. not noticable to the human eye.. maybe NVidia has a bit better color depth for DVD's, but running 2 identical systems side by side one with each of the best cards by either company, the movie looks identical to the human eye..
basically I'm just stating that both cards are good and as such, it boils down to user preference.. Both cards are about the same price so it is really a personal call.. however performance wize, the 6800 wins by 10% currently across the board, however the x850, ATI's newest has the new refignments that the 6800 Ultra brought out, only it is just a tad bit slower in the GPU speed in rendering, but not enough to see a difference even in your most graphical intensive game...
Anyhow, like I stated.. they cost about the same.. go with what you are comfortable with..