I don't know how you'll go gaming across *two* monitors. You'll get a split right down the middle due to the bezel. Most people go with 3 monitors so that you get your character/car/weapons in the middle monitor and then peripheral vision on the sides.
I haven't found any reviews on the 670 where multi monitors are used, but by halving the frame rates that you get in 1920x1080 games you could expect 30-40 FPS in most demanding titles, 50-60 in less demanding ones. 670 review at Tom's.
The 680 will do it better, but not astoundingly better (since there isn't much difference between the 670 and the 680).
You can expect the 690 however to run very well (since it's two 680s on the one PCB), but if it's worth the cost is the question. You'd be getting 60+ FPS in everything pretty much.690 review at Tom's.