With all advanced settings turned on, and the resolution set to 800 x 640 at medium quality I get 35.5 FPS average on a timedemo run. When I increase the resolution to 1024 x 768, I get 26.5 average.
P4 2.0GHz
512MB 2100 Ram
9600 XT (Catalyst 4.8, overdrive enabled)
You should be able to run it on similar settings and performance. There are tweaks that is said to increase the performance quite significantly. I have yet to try these tweaks, but I still get a satisfying 35.5 FPS. I heard the new patch from Idsoftware will include some of the functional tweaks, so I'm waiting for that. Enjoy the game.