I don't know much about high end video cards but go to a website like: http://www.anandtech.com/bench/GPU12/372
and see which is faster and if it would make a difference. If you are running on large monitor(s) then go with extra gbs. Hope this might of helped.
670 is a good choice. I would go for the 3gb version if it's only a couple bucks extra. As for what brand to get for your card it doesn't matter a whole lot. Most brands have about the same build quality.
Unless you're playing at a resolution higher than 1080p, don't bother with 3GB VRAM.
Also... the next line of video cards will be coming out in a few months. Unless your card died, I'd wait till then - these cards will be cheaper, and the new ones are going to have a fairly sizable performance boost.