I have no idea why many people(not pointing fingers at anyone here, but in general) treats Doom 3 as a game that requires performance so immense that it's way out of Xbox or GameCube's league(putting resolution aside).
We are still very early in the consoles life, and developers are pretty much still scratching the surface of their potential. Developers still have yet to bring those consoles to their knees, and even when that happens, they'll continue to squeeze every drop of power out of them by employing neat tricks and writing unique, more efficient code.
Inevitably, there will be games on both those systems that are more technically demanding than Doom 3...although, whether they look better or not may depend on if they use similar techniques as id's engine-- simulating incredibly complex geometry through the use of textures("polybumps")
PS: That's why I'm eager to see Factor 5's next engine/game. Their first GC game(Rogue Leader) threw around much of the graphical features *most* developers are only NOW implenting in their games(per-pixel lighting, self-shadowing, pixel shaders, 5+ simultaneous textures, etc). Certainly, they are watching how the competition raises the bar each time, and will also naturally set out to make RL look like a complete joke . I'm anxious to witness the results their "second day out" with the NGC.
