Ok, listen I can tell you that a Ford Focus can beat Lamborghini Murcielago in a race. If I dont show any proof, that can be true or not true.
Why don't you read some benchmarks and see for yourself how each card performs in games. In Call of Duty and Doom 3, 6800 series will win hands down. Also 6800 series dont do bad in counter-strike so I dont think ATI will have any significant dominance in HL2. ATI won when Nvidia drivers were unoptimized, but Nvidia is keeping things close.
Here are some benchmarks you should look at:
Digit-Life (More recent drivers)
- Nvidia wins HL2 here
- Nvidia wins counter-strike too (based off HL2 engine) =
FiringSquad
XbitLabs (More recent)
The problem is that ATI did dominate in some benchmarks (like Far Cry, HL2, etc 6 months ago, but Nvidia drivers made things much closer) Pretty much 6800Ultra and X800xt Pe will offer equal gaming experience but ATI cards suck in Doom 3 (so expect them to suck in Quake 4 and all other games based off Doom 3 engine). Also different review sites show different information (you can find sites that show Ati winning HL2 and Counter-Strike). Make sure to note that often a card might have a higher average frame rate, but lower minimum framerate (thus actually offering an inferior gaming experience).
Nvidia cards also let you use HDR (albeit at a huge performance loss).
If i was you, i'd just get 6800GT for $380 because the price of admittion for X800xt PE/6800Ultra is outrageous for the mere performance improvement. 6800GT should get to Ultra speeds to.
Here are some older reviews with somewhat outdated drivers:
VGA Charts IV
Anandtech
Fastest Cards of Summer 2004
A better way to evaluate the card is not see which cards dominates in more games than the other. Instead you should be concerned with the performance of the cards in the games you play the most.
But if you want the ultimate verdict. X800xt pe is better for the majority of the games but its value is questionable, especially when refresh products will come out in December, possibly forcing a reduction in pricing from the outrages 500-600US for x800xt pe cards. So why pay that much now, when in 2 months it'll drop severely? Same applies to 6800Ultra.