Are you saying PC version (which is a port of the PS3 or XBOX version) uses 256MB?
Not ranting but Doom 3 was written for PC and it uses the id tech 4 opengl engine it is not a port from any console and it was released 2004 so it was made before ps3 and 360 existed.
Also there patch so it can run on 2x sli voodoo 2 cards with 12MB vram but it looks crap but is quite cool since it needs 64MB vram

Just trivial facts.
I understand that some need more ram on their Amiga but I am happy with 64MB
But more would not hurt but I'm not in need of more atm.