Right from the Doom 3 FAQ:
ATI's Proprietary Linux Drivers (fglrx) version 3.14.6 and up will run DOOM3. Download fglrx from http://www.ati.com/support/drivers/l...eon-linux.html.
The DRI drivers for ATI cards may run the game with some tweaking ( you need to apply the S3TC patch for instance ). But the rendering quality will be sub-optimal, as a number of important graphic features are not available in the DRI drivers.