Hi all. I had the same FPS probs as others have reported: I was able to play for a while with decent FPS, but then it all went to hell and started chugging along, especially in the third mission. I tried some of the suggestions like playing around with graphics settings, rolling back drivers, following the ARMAholic's optimization guide, etc and nothing seemed to help, but then I decided to download the current build of Windows 7 64bit (I had been playing on Vista 32bit) and up my system memory from 3GB to 4GB and it worked like a charm!!! I have been playing for a few hours smooth as butter through parts that chugged like hell previously.
I haven't benchmarked the fps to give you hard numbers, but the game runs like it should. I have all the settings on high, except for shadows, which is on "Normal". View distance is 3000. Res is 1680x1050. I will play around with higher settings and see what happens.
I am sure the extra gig of memory is helping, but, somehow, I think the Windows 7 RC has more to do with the smooth framerate. I don't know if this message will help anyone, but you might try Windows 7 if you're currently on Vista and having FPS, as it may help. The key here may be that, compared to Vista, Windows 7 is less demanding on your system resources.
In case you're interested, here are my specs:
AMD 64 X2 6000+ 3.0GHZ (Stock Speed)
4GB RAM
Windows 7 (Current RC Build) 64-Bit
EVGA GTX 260 (Stock Speeds); 186.18 Geforce Drivers (64bit)
Western Digital 640 GB HDD
Cheers!