I've played a little of the PC & 360 versions (I own the 360 version and have access to the PC version). Thankfully FarCry II isn't a resource hog like Crysis.
If you meet just the min requirements you should float around 30 to 40 frames in the open areas. From poking around the net I've found some people with lower end systems notice the game drops to 20 frames in certain areas and the game runs better on Vista systems than Windows XP.
ATI recently released a hotfix for the game, it "Improves performance on Crossfire and single-card setups, in both DX9 and DX10." Rumor is Nvida is going to release a patch too.
Also, be aware if you have a widescreen monitor the game crops the full screen image to create the widescreen image. There is a fan made patch out there that makes the game run in true widescreen; nothing official from Ubisoft yet.
Lastly, taking a trip to
System Requirements Lab and using "Can You RUN It?" never hurts if you're worried about your specs.