I went from 1920x1200 ASUS VW266H to a 2560x1440 Shimian IPS.
Night and day difference in terms of color, sharpness, etc....you name it. Arguably the best $390 i've spent on PC hardware in the last 2-3 years. You can even get my monitor for like $340 now...so there's no question it's worth it.
That said, expect a single 680 to run Ultra w/ 0xAA @ 2560x1440 at between 40-50FPS average. I get about 57-58 avg. on my OC'd GTX 590 w/ 2xAA. 4xAA instantly kills my card, because the VRAM wall is hit so fat at times, mostly on the huge, outdoor maps.
Make the upgrade, and sit back and be amazed at how awesome it is to step to beyond 1080/1200p - you won't even be able to look at lower res monitors anymore.....I know my 3x 1080p's at work make me sick, lol...