A GTX680 is just a higher binned 670 plus some extra CUDA cores, there is a minimal performance difference between the two. You can quite easily overclock a 670 to beyond stock 680 performance.
Go for the 670, or even better, a HD7970 Ghz. Radeon cards just perform better then Geforce when your at a high resolution.http://www.newegg.com/Product/Product.aspx?Item=N82E16811235027
Have a look at the 2560x1600 result, 10% performance difference between a 7970 Ghz and 680. 5760x1080 (which is three 1080p monitors) is double the amount of pixels.
But regardless of which you go for, you will need dual cards to get good performance at higher settings.
750W is enough for dual cards, and dont go for Coolermaster PSU's.