What monitor do you have in 3840x2160x60Hz with the GTX 680 ?I'm currently driving a 2560x1440x60Hz off the DVI and a 3840x2160x60Hz off the display port of a GTX680 (inside mac osx). So it works just fine for 4k.
Actually better than the ATI7950 - you can do 4k off that too, but SST monitors like the Samsung get corruption above 50Hz due to a driver issue. Unfortunately affects all the nMP owners too.
EDIT: While the Samsung 28" is good for the price, I wouldn't recommend it in general. I actually prefer my Seiki 39" even though its 30Hz restricted - colors are viewing angle way better, and if you want to run at 1:1 the dots are actually acceptably large. The samsung gets you 60Hz, but other than that I'd consider it inferior.
And is your GTX 680 a Mac Edition, it seem the Mac Edition does not support more than 2560x1600 in the display port :
Well, it run 4K on windows but not on Mac OS, it cost 100$ more than the regular version and does not provide the same functionalities... ?
I've read a lot about the Seiki 39", that's what drive me to buy the 30" DELL.
But I've also read that the 30Hz is really frustrating with a lot of lag with the mouse...
Also, is 39" not a giant monitor for a working usage ?
As you have a 4K monitor with Mac OS, does the fonts are normally readable, or is everything very little ? Do you enjoy the very large resolution for working with ?