For me, I think the 6000 series is remarkably good. I picked up a 6800 non-XT for something like 380 dollars late last year. Personally, I think that's my suggestion if you plan to move to 1440p (that was my reason for upgrading as well).

IMO 8 GB VRAM isn't enough nowadays, and I suspect 12 GB is next on the chopping block. It may be barely enough for now, but I don't think it'll be enough in the near future. Nobody can predict the future, but I would honestly go straight for 16 GB VRAM cards if you want your card to have any lasting power