I'm running on a P4 2.8 with HT
512 ram
128 Radeon 9800 Pro
and I'm running at 1024x768 in high detail. I get the occasional stutter when opening a new door, but not all that often.
I have to say, I did increase my performance highly after turning AA and AF in the ATI control panel to "Application Preference" and then making sure AA was turned off in Doom 3. I'm sure running at Medium Detail on 1024x768 would be no problem at all.