I think if you want to play games with single monitor setup than even 2GB VRAM will be sufficient for future. And I prefer a single powerful GPU instead of SLI/Crossfire.
If you are thinking that you can multiply VRAM then you are wrong. Ex. 2 x 770 with 2GB VRAM = 2GB VRAM, not 4GB VRAM.
I know I can't multiply ram, but you can buy a 4gb version of the 770. I will go one monitor 1440p, and maybe a second later.