The intention of this forum is to answer a question that was in the middle of productive discussion until it was wrongfully deleted. I myself am giving my vote of confidence that a helpful discussion will endure by strictly posting things that I think are helpful to solving this issue.
Building off of this, sid, the toxic and vapor x have 6gb versions so that advantage would actually go to them although 4gb is likely enough even for 3 monitor resolutions. Unfortunately no benchmarks exist between the two as you would need a triple 1600p monitor setup with lots of mods and AA to max out 4 or 6 gb.
The issue that personally worries me more is if the smaller memory bandwidth of the 680 is enough to make use of all 4gb effectively.