A GTX 590 is nearly useless with a single screen, in my opinion. A GTX 580 can nearly max out any game in 3D at 1920x1080. Really cranking up the AA could maybe give the GTX 580 a hiccup if you don't OC it. I'd get a couple of screens myself.
But to more directly answer your question--I'd go with the 27" ASUS VG278H over a 24". Besides, that ASUS is supposedly a great screen (I haven't seen it myself yet).