actually i'm also using a 680gtx kepler.
and although i'm only using it to power a 1920x1200 resolution 24'' u2413 ah-ips monitor, i saw the performance suffer in Dragon Age Inquisition at ultra settings (aka unplayable).
Had to lower settings to high for fps of 40 ish.
That to me is a sign i need to upgrade soon
This is why i don't really want to jump from 24'' to 27'', because the higher you go the more gpu intensive it becomes which only leads to having to get a sli solution which cost a bomb

Also i find 24'' more than sufficient for me. Not to mention that most of my media is either 720p or 1080p, which works best at 24'' anyway. If i go higher it just scales up and the picture is less sharp (though probably not such a big issue these days i guess for bluray content).
although for me 4k is still a ways off (because waiting for more media to be 4k before jumping onto that bandwagon), but is interesting to see what fps numbers we can expect come pascal and volta for gaming at 4k and still get fps of 40-60 or even higher.