For just one monitor? A GTX 680 would be more ideal, and it'll do everything you need it to do. Also, this future-proofs you as, in the future should you decide to run more monitors you can add another GTX 680, they'll be easier to find then, and be able to smoke any game/3D out there.
I dont really get it but your getting Multiple Monitor setup? If your doing two monitor setup, get two GTx 560 because a single 680 at 1920x1080(1 monitoe) with crysis 2 running max is in the low 30s average FPS(33FPS)... Best if you get two 560 Ti 2GB because it probably could run better than 680 due to low VRAM of 680... You can wait for the GTx 680 4GB vRAM which is ganna be awesome or Multi monitor Setup which should be probably or close to $650 as my guess