Look it's basicly not as simple as you want it to be. In general the 6800 Ultra will have a slight performance edge in Doom3 and the ATI x800 XT will probably perform better for Half-Life 2. Both cards will work very well and give excellent performance with both games.
As it stands at some resolutions the 6800 is about 10 fps faster than the x800 XT, but I expect that gap to close a little with the new ATI drivers. Still because of the shadow stencils on the 6800 they will always have a small edge in Doom3.
Another thing to ask yourself is is your power supply up to an 6800 Ultra these cards are real power guzzlers. the 8* AA mode is almost useless and unplayable in any modern game. the 3.0 Shaders are little supported so far but could give this card an edge once it is adopted in a more wide spread way by game designers.
Having spent 2 month reading ever review, comparison and article on both cards and their variants I've gone with the smaller less power guzzling ATI card which has the slight performance edge in most games. Also this card goesn't have a fan so large it requires two slots in your PC.
As it stands at some resolutions the 6800 is about 10 fps faster than the x800 XT, but I expect that gap to close a little with the new ATI drivers. Still because of the shadow stencils on the 6800 they will always have a small edge in Doom3.
Another thing to ask yourself is is your power supply up to an 6800 Ultra these cards are real power guzzlers. the 8* AA mode is almost useless and unplayable in any modern game. the 3.0 Shaders are little supported so far but could give this card an edge once it is adopted in a more wide spread way by game designers.
Having spent 2 month reading ever review, comparison and article on both cards and their variants I've gone with the smaller less power guzzling ATI card which has the slight performance edge in most games. Also this card goesn't have a fan so large it requires two slots in your PC.