I've had no problems so far gaming at 2560x1440 with 3GB but I'm very conscious that often I'm running it quite close to the limits - which is why I haven't gone 780 SLI.
If as he said its gonna be past down though then there is a good case for going for a little newer card.
'snot newer though, is it? it's just a rebadged 290 with slightly better manners and 8gb VRAM.
I wouldn't pay over £185 for that.