Well really the GTX 670's are still very good GPU's. Besides a higher Vram speed and Boost 2 the GTX 770's are still based on the same silicon as the GTX 6xx cards. In all respects you would be just fine with the GTX 670's.
If you're gaming at 1920x1080 or less, the 2gb cards will be fine. If you are going higher resolution, then the 4gb cards will make a difference. As far as future proofing, there is no such thing. IMO, buy the best SINGLE card solution you can afford. SLI and/or Xfire drivers issues (especially Xfire) make multi cards a non-starter for me.
Two 670's will run you about $700 - you can get a 780 w/3gb and a 384 bit bus for the same price.