I just bought a gtx 670 and fired it up to see what difference it would make to my old gtx 260. The first thing I tried was ArmA 2, and did a benchmark to compare it to another one I saw with the gtx 670 on the internet.
That guy got an avarage of 69 fps, and I set it to exactly the same settings, but on 1366x762 rather than 1920x1080.. and I turned out to get whole 35fps.
CPU bottleneck. I also have a q9550(soon to be replaced) and it bottlenecks my gtx 580. If you do a cpu upgrade you'll need a motherboard and possibly memory aswell.
i have evga gtx 660 ftw signature 2 3gb and seems this is top what q9550 @ stock can balance with. i am getting 95% cpu/gpu usage. maybe i would be able to pull off 660ti but yeah gtx 670 is probably too much for stock q9550