Ultra quality sets itself fine on my gf 6800GS 256 mb on both q4 and d3.
Both games give some kind of warning but after pressign ok they run ok.
Quake 4 gives vram lag when loading new lvls when opening doors the 1st time, smooth otherwise...
Doom3 is lag free & smooth.
I don't know the ati situation, but i know nvidia is a bit better with opengl as ati comparable cards, and that quake4 & d3 like high cpu frequency's (ran better on p4's than a64's).
But with specs in sig your they should run A-ok, provided the gfx has 512 mb of vram, if not q4 might show a stutter each time when opening a door.