i've heard that omega drivers increase performance dramatically in doom3. you can also use the humus tweak (search google) that should give you an fps boost. the omega drivers can be found here: http://www.omegadrivers.net/ . enjoy.
ps: i myself have tried the omega drivers, but hadn't done any doom3 playing before. i havent personally noticed a performance increase with my 9600xt, but its worth a shot.