Red Faction Guerrilla Constant lag on gtx 260

hi, I need some help im identifying what component in my computer is, without a doubt, faulty.
MY specs are:
GTX260 (possible suspect!)
4gb RAM
Quad core Q8200 (possible suspect!) (typically easily good enough for this game)
Windows XP home (Also, about 15fps less on windows 7)

Basically, Something strange is going on, I get at MAX 40 fps (and that's when I'm looking right down onto flat ground, and even then for some reason it has a really... flick lag feeling, which makes me think it could be the cpu constantly struggling) and an average of 30fps while running around and driving cars (and I know what 30fps is like, and this is WORSE than 30fps), if i look anywhere where there's something interesting going on, it drops down to 20fps. Do you think it's my processor or Card? Thanks.

(Also, I cant, on any resolution, smoothly watch 1080p HD films for some reason, even though a gtx260 is far great enough)

No components are Overclocked f.y.i

It runs a lot smoother (but still, ridiculously, not perfect) when I lower the resolution to 800x600. I see other people with worse computers than me saying they get a constant 50fps on highest settings  
HELP!!!!