What is your CPU? Crysis 2 on a single 680 gives me 60 fps 95% of the time, and never goes below 50 at max settings. I am very much a stickler when it comes to high FPS, which is why I have 2 680's. Your 670 is only going to be faster due to OCing, which is also something I did with 1. With 2 680's, I don't bother, though it may help get a little more for Metro 2033.
Edit: When was the last time you played Crysis 2 in 3D? The reason I ask is that 9 months ago, when I played it, it got much lower FPS than a couple months ago. There may have been a patch or driver change that radically improved performance.