"Future-proofing" your system by having more VRAM will not be as effective as having more processing power. You will be better of getting a better card with the same/less memory (780 ti will get you farther in the long run than a 6GB 780). But from what you are saying, an aftermarket 290 or 290X would be appealing to you. Also, if you can wait (I understand if you can't, I couldn't wait to build mine, ahaha) you should wait for the 800 series, as they will perform better for a significantly lower price than the 700 series.