Well its personal prefference kinda...
ATI will get slightly better FPS is alot of first person shooters but the drivers tend to be not the greatest....
Nvidia has excelent drivers, are very stable and are alot cheaper 50-100 bucks cheaper...they run better in some FPS such as FarCry(I think) and Doom3...also many games are developed on them wich is a plus...

If I were you I would go with Nvidia as it may (im not 100% on this)perform better in FPS, the difference in fram rates between the ATI and Nvidia and detail will be less than 5 FPS + or - difference and detail the same thing, hardly knowticeable....Nvidia will OC better also.... so if you must have the the ATI thats gonna get you those extra 5 fps in some games and you want to shell out the cash go for it....but as far as I am concerned ( I don't want to cause a flame war here) Nvidia is the better product, it will OC to get the extra FPS of the ATI and OC farther than the ATI....so if you have 600 to spend the Nvidia option is  Chaintech Power Now! 6800/2600 Ultra(air cooled or watercooled) that card cost more because its clocked at 450 core/1200 RAM Your ATI option in that price range would be somethng like the Powercolor ATI Radeon X800XT Platinum Edition with clock speeds of 520 core/1120 Memory, so you ask if ATI is clocked that much higher why dont I get that well because the ATI has less Pixel Pipes than the Ultra, not shure how many less...P.S. with ATI you get VIVO, Nvidia you do not

500 dollar Range... Nvidia there is something like the BFG 6800 Ultra OC wich has clock speeds of 425/1100Mhz, it should OC to 450/1200 pretty easy....this is a very popular card..

ATI there is somethig like the   GIGABYTE ATI RADEON X800 XT wich is clocked at 500/1000 also has VIVO...

So its kind of personal prefference, comfort and what you plan to use it for... NOTE: These cards are AGP and have 256 Mbs GDDR3 memory.... Any one feel free to add on to this as im shre I have missed a lot but my final say is for the best card/bang for the buck would be the Nvidia...

When you all get your 6800Ultras please remember to turn off fastwrites in your BIOS.  I think a few off the  Ultras are ok but with most you get serious stuttering if fastwrites is left turned on!  Try it out thats the best way.  I did and was getting 2-8second pauses with it on!!