Dude, You cna't play Doom 3@Ultra Quality on a ATI 9000 Whether you use 8GBs Of Ram or Not, Doom 3 has the most advanced graphical engine to date, in Ultra QUality the texels are uncompressed Thus they require a massive 512MBs of Video Ram, and Doom3 also requires atleast 6Pixel and 2 Vertex shaders to work without jerking
if you want to play Doom 3 at Ultra quality you cna either buy a ATI X850XT which has 3Dc a technology in which textures are compressed to work load off the main Memory Chips, so hey don't overheat 8) on the other hand get a nVIDIA 6800GT or Ultra, or even better a 6600GT, it offers superb performance with tasty price
at 11.5k you with be able to play D3@High Quality with AA and AF and ge a smooth Frame Rate