Those are pretty good framerates, but then again it's on 640x480.. have you tried higher resolutions?
Yeah, I know, 640x480 windowed is just what I run most of the time, so the game doesn't get in the way when working on it.
With the same settings @1600x900 its 79fps on all three runs.
On even bigger resolutions it starts to slow down to a crawl, but I didn't look into that. Maybe a mesa bug, maybe filled vram, dunno.
In any case, r600g makes doom3 more than just playable.
I doubt that the slowdown on high resolutions is due to a filled VRAM, it's got 1GB, and assuming the driver reads it properly and use it, it'll take way more than Doom 3 at high resolution to fill it up, I think