The 4GB version of the 760 is rather useless as it won't really provide the necessary GPU performance for a resolution that would require 4GB of VRAM. Even in SLI.
If you're aim is to play games at High, the 2GB version of the 770 is something that fits the bill exactly. The card will allow you to play at high, and you'll have more than enough VRAM. And if you choose to SLI the 770 in the future, 2GB of VRAM should still be more than enough unless you decide to try to push the limits of your GPU(s) by playing on maxed settings.