I just had a look at a 670 review and it seems to perform well at 2560x1600, i just want to be sure that it will run games like Crysis 3 and Hitman absolution at 1440p on high settings.
also, i have seen a 2nd hand 590 going for near 300 quid... worthwhile? I like the extra VRAM it offers, but not sure as it is an old card.
Realistically it offers 2gb of vram.
The 670 is better than a 590 in heat and power and about the same in speed.
High on 1440p might need 2 670's. They perform about the same as oc'ed 680 when it's oc'ed. You won't be disappointed with its performance. I recommend the Asus dcuII as it's cool fast and quiet.