As Gam3r01 said...no real thing as future proof. What I have found is best, get a GPU that performs as you need (and not overkill) and if in a year or 2 a more demanding game comes out then sell the GPU and upgrade. Case in point.
I had a 660ti and it maxed out BF4. It cost $315. At the time the 7970 was $450-500. And it was slight overkill for 1x1080p gaming. However, a year later the 280x came out (essentially a 7970) in time for BF4, and I got this GPU for $315 as well. I sold the 660ti for $225. So, I did take a bigger hit on this one (compared to last GPU which buy and sell price was only $25 diff) but I have newer product and warranty. Seems like win win in my book.
My advice. Buy a GPU for intended purpose based on what you need now. You can always upgrade in the future, but there really is no point to a 6GB GPU.