The 4GB per GPU cards are mostly a marketing gimmick for Kepler, at least as far as the memory capacity goes (in fact, having more memory capacity is usually a slight performance decrease if not for the factory overclocks). It's not until three-way and four-way SLI where it makes any difference at all and even then, the difference is negligible. They generally don't have enough memory bandwidth to run well when the 2GB models run out of memory capacity. With overclocking, that might be a little different. but even then, no single card will be able to overclock far enough for it to matter and even two cards probably couldn't. Maybe three or four well-overclocked 670/680 cards in SLI could run games well enough for 4GB of graphics memory per GPU to be helpful.
If you want a great Nvidia card, my suggestion would be the Zotac AMP! GTX 670 if you want to play at stock. IT'll beat most factory overclocked 680s regardless of their factory overclocks thanks to it having not only a significant GPU frequency factory overclock, but a much more important memory frequency overclock too (something that, ironically, is overlooked in most other Kepler cards despite being more important than GPU frequency boosts for most of the Kepler cards).
Oh I see you have a 120 hz monitor. I can't imagine you not being able to have really great frames on high with BF3 given that Ultra is the maxed out level of graphics. I'm with Jay if you would like more frames adding another 660Ti would the be the best option economically and performance wise. I'm sure BF3 would react very well to SLi performance. With a 120 hz monitor I'm sure you could get 120 FPS with two 660Ti's in SLi. Given your system specs with ease.
Your programs will turn into a laggy mess far before 4GB is all used because of the extremely restrictive 256-bit bus. And that's assuming 4GB is used, most games don't use more than 3GB, or even 2GB.
Add to that the cheaper 7970GHz is faster than the GTX680 and can overclock to even faster than a GTX590. And I would reccomend the 7970GHz instead.
However if you want the Classified is the best, albeit the most expensive, custom cooling, custom PCB, and factory overclocks, the DirectCUII 4GB(It's actually dual not triple as said before) is close behind with DIGI+ VRM, etc..