Having read the 3 pages of this thread, it seems that noone really wants to get down to the basic issue.........dropping the resolution results in no differene whatsoever to the rate. Whilst a few people think its a CPU bottleneck (and that's a hell of a bottleneck, let me just add a "me too".
I'm running an OCed E6600 (3.4Ghz, granted not the fastest or most recent processor out there) with a GTX275 in Vista x64. I can set solution at 1680*1050 with all settings on High or Very high and get exactly the same FPS as when I choose 800*600 and all settings on Low. Seriously, never budges off the 21-22 FPS (drops to 15-16 intermittently). View distance set to 400-500.
I have never seen anything like that. By all means, tell me my CPU is a bottleneck, but I should be able to *SOME* difference at the very low resoution and settings. Its frustrating as hell, and unplayable at this FPS......