The choice is ultimately yours, but one thing you want to consider is price. Since you said there are only 3GB versions of the 7950 sold in your country and you already have one, a 27" monitor will work fine.
Smaller IPS monitors are usually around $200-$400 (Ultrasharp models are reference on Dell's website). I'd go with a 27" Korean monitor. They might not have the best contrast or something of the sort and have a small risk to them, but people usually end up getting good monitors.
I can't really take advantage of a monitor that size, and I'm going to be sitting pretty far from it and because of my horrible eyesight, I can merely see the text, and I don't have $400 to spare so a smaller monitor is fine for me. I use a small 15.6" monitor all the time (laptop in my sig) and a 19" 4:3 NEC I found laying around. I've seen a 21.5" 16:9 monitor and it felt big to me, so that would be more than good enough for me, but this is just my
and you may think differently.