Apply the latest Doom 3 patch v1.1 in the first place. Get the latest Catalyst drivers and install them, they give ATI users a good boost with framerates in Doom 3.
Then follow the guides below in the respective order in which they are posted here:
Windows XP Tweak Guide:
*www.tweakguides.com/XPTC.html
ATI Catalyst Tweak Guide:
*www.tweakguides.com/ATICAT_1.html
Doom 3 - Game settings and tweaks:
The ones marked in bold are what will boost your framerates maximum:
*www.tweakguides.com/Doom3_5.html
*www.tweakguides.com/Doom3_6.html
*www.tweakguides.com/Doom3_7.html
*www.tweakguides.com/Doom3_8.html
*www.tweakguides.com/Doom3_9.html
*www.tweakguides.com/Doom3_10.html
Doom 3 Performance Tweaks and Tips:
These will get you a good performance boost too
*doom.neoseeker.com/DOOM_3/PC/pages/performance_tweaks/
Finally, download GameGain from here and run it to optimize your system:
*www.majorgeeks.com/download3930.html
I hope these will fix your experience with Doom 3 a bit.