I'm averaging around 30 FPS at 800x600 on medium-high settings, XP of course. I game on a 15' CRT screen so I don't notice a difference with a low resolution.
1280x1024, everything set to high, except postprocessing detail and shader detail, those are on low, with that i get an average of 45-50fps, sometimes it drops to 35, sometimes it goes to 70 tough. ofc, thats DX9, but it'll suit me ^^