You can only run with ultra detail on vid cards that have 512MiB of RAM, regardless of whether it's a 6600, 6800 or 7800.
Just for a laugh, I enabled it on my 7800GTX and it rendered the game unplayable due to the memory thrashing slowing everything down to a crawl.
Needless to say, I run at high detail 1600x1200. The only annoying thing is Doom 3 and Quake 4 being limited to 60fps maximum, so when I play it on an 85Hz display, it suffers from almost (but not quite) subliminal stuttering.
Just for a laugh, I enabled it on my 7800GTX and it rendered the game unplayable due to the memory thrashing slowing everything down to a crawl.
Needless to say, I run at high detail 1600x1200. The only annoying thing is Doom 3 and Quake 4 being limited to 60fps maximum, so when I play it on an 85Hz display, it suffers from almost (but not quite) subliminal stuttering.