I can't imagine gaming on a small screen anymore. I'm so spoiled. Why not get a 40" tv and use that for a monitor?
I gamed on a 1080p 37" for years and talked myself into being mostly happy with it. Sold it because I was moving cross country, plus it was getting old. Bought a new top of the line 40" Samsung 1080p thinking that if I wasn't 100% happy with it I'd just move it into the bedroom and get a 1600p monitor. I wasn't 100% happy with it, and now have the 1600p, and now I'm kicking myself for not doing it years ago.
1080p and 37-40" at normal desk viewing distances just doesn't cut it unless you're very tolerant of crappy pixel pitch. The biggest I'd even think of is 32" for 1080p but that sucks massively compared to 30" at 1600p. The only "TV" I'd think about using as a PC monitor (i.e. sitting at a desk, things change once you're sitting on a couch) now is a 4k.
I have been a fan of SLI for years but these days SLI is becoming diminishing returns. Unless you get your cards used often one current generation will outperform dual SLI configurations of one or two generations back for less $$$. In reality the CPU and RAM become the bottle neck before even one modern video card.
I'm not sure whether or not the "buy one now, SLI later when they get cheaper" argument ever really was true. I know there were several times I intended to do it, and then never ended up doing it, probably precisely for the reason you cited.
Where it does (or at least can) make sense is buying them both at once. It may not be quite the same anymore with the recent nvidia price shake up, but 2x770s as opposed to a single 780 made a lot of sense in that you got something like 40% more performance for 20% more money.
The other is when you just flat out need tons of horsepower and no single card will cut it, e.g. running 3x1600p monitors, or a 4k monitor, or maybe a 1440p @ 144Hz.
To the OP, as has been pointed out, you won't get any benefit from the extra 2GB of the new card. I'd really try and pick up a 2GB used, that's where your best value is going to be. 2x670s gets you right in the thick of things in terms of modern single card performance (should be better than a single 780).
I haven't gamed at 120/144 Hz but a lot of people rave about it, and you could probably use some more horsepower even at 1080p if that's what you're shooting for, especially on 2 (or 3?) monitors. Can always pick up a game and see how it runs and then upgrade if you're not getting the framerates you want, though if you're thinking about buying a new card make sure you don't buy a game that you could have gotten for free with the new card (nvidia has a lot of free game deals going on right now). I'd recommend Bioshock Infinite or Assassins Creed 4.
