The only game that I'd ever been able to see top out my old GTX 580 1536 was BF3 at 2560x1440. 1080p may be the standard gamers resolution today but the reality is..its a pretty low resolution compared to what high end cards today can really push. Frankly I think many would agree really all you would need to do 1080p on high details in the vast majority of games would even be a 560TI, which you can get for around $250ish with rebates.
But it's his money and if he wants the fastest card, that would be the GTX 680. The extra ram on the 670 isn't going to make up for the price premium you're going to pay (which looks to be around $100 vs the 2gb model). The fact that Nvidia launched the 680 at 2gb instead of 4gb should be some indication that they didn't feel it was worth it either.
Future proofing isn't really going to be as big a factor as you think to be honest. By the time you find a game that requires 4gb of ram to run 1080p, the 670 architecture is going to be so out dated that having the 4gb of vram wont make it any more playable than only having the 2gb version. But I suppose people running SLI set ups with multiple monitors would benefit from the extra ram, and being that the 670 adn 680 are close in performance, and most people would likely run 670s in SLI given their price...it is the obvious choice to make a 4gb version. But single card, single monitor...I don't see 4gb being any faster.