Being so close to the 680 for nothing but a memory bump i would save a few more clams and buy a 680.
Nah, the GTX 670 performs so close to the 680, a 680, especially given low availability right now, is a poor value proposition. If you really wanted something a bit faster than a GTX 670, I'd suggest the HD 7970 as it can be had for around $450 if you look around - so not too much more than a 670, and given average overclocking headroom, will get you a bit more performance than a GTX 670 if gaming at 4MP (ie, 2x 1920x1200 or 2560x1600). Also the higher memory bandwidth on the HD 7970 helps a bit at higher resolutions.
But for one 1900x1200 monitor the GTX 670 is even a bit overkill. The GTX 670 is better at folding than the Radeon as Folding client is optimized for CUDA. However if it ever becomes optimized for OpenCL, the HD 7970 should be superior as it has significantly more GPGPU compute power.
Since you have one 1900x1200 monitor now and are thinking about getting a second, and given the games you listed and current folding performance, I think the GTX 670 is a good choice and as much a card as you'd want to go.