Will it really run out of bandwitdth? That is extremely misleading on nvidia's part. How much actual vram would you say it could have before running out of bandwidth?
It's not Nvidia's fault IIRC, Nvidia didn't market the 670 4GB AFAIK.
There are a few situations where the 4GB model makes a difference, I admit that. However, there aren't many and in them, AMD is usually the better option anyway.
I'd say a little more than where 2GB is a bottle-neck is where it usually really starts to struggle, but AMD's lead at that point is usually too much to pass up IMO.