The performance gap at higher resolution is irrelevant when 1080p is the intended use. However, I realize that ppeoplecchange monitors fairly often, heck I switch between 1080p, 1200p, 1440p, 1600p, and others (30" U3011/3014 landscape with two 1600x1200 portrai, or 1920x1200 landscape wwith two 1600x1200, or even 1440p landscape with two 1440x768 portrait) quite often.
I don't use 120hz, as I absolutely cannot stand the horrific color, tremendously terrible contrast, and all the other sacrifices needed to use lightboost, and regular 120hz isn't worth having to use a TN panel as my main display. Fortunately, I have almost all of my monitors running at 75hz without any issues or frame dropping. I have to give credit to Monoprice, as I was unable to run higher than 63hz on any display using decent DL-DVI-D cables, but the instant I switched to their extremely thick and well insulated Dual-Link DVI-D cable, 75hz has worked with zero issues, and I can get close to 96hz on a couple of displays. The only negative is that the cable is extremely thick, thus extremely stiff, which is not a problem for me but I can easily see how it would be for some.
I think that the best thing to do is to always buy the absolute best single GPU you can, and then if necessary, SLI later. Scaling becomes an issue at 3+ cards, so I would have trouble recommending, for example, that anyone buy two 760s with plans to add another, in fact tri-SLI 760s show NEGATIVE scaling in a few titles! 3 way 770s, while an improvement over two, will expose and be held back by any other bottleneck in your system, while generating more heat and consuming more power to deliver essentially identical performance to two 780s. The maybe 10pct gain of 3 770s isn't worth the heat/power/compatibility issues/etc over the better 780s.
If it was my money, I would wait for and buy the MSI 780 Lightning, assuming it's as good as it should be. Running a pair of 680Lgt in SLI in my benching rig, water-cooled with the phenomenal Aquacomputer blocks (in same loop as delidded 3770K @ 5.1-5.4Ghz, XSPC GA-Z77X-UP7 motherboard VRM block, and an XSPC 4-DIMM universal RAM block modified to fit the 2x4GB Trident X 2400CL9; MCP35X2 pumping, UT60 560 w Bgears Blasters push+pull w Phobya shrouds, EX420 with the same fans/shrouds, and HWL BIX 360 with Koolance 120x38mm 2600rpm 117cfm/6.3mmH2O fans P-P w Phobya shrouds all cooling).
I have had the 7970 Lightnings run both alone and in CFX, same system, and as much as losing them to faulty blocks sucked, I am so much happier with the 680s, recent drivers excluded.
Basically, the 7970 Lightnings didn't clock that much higher than regular 7970s even with a fair amount of extra voltage, reaching mid 1300s but not 100 percent stable. The 680s are sitting at 1484Mhz core and 7196Mhz memory right now completely stable, and will run in the 1600s benching with a bit more room to push them.
The performance difference is not insignificant, but what numbers don't tell you is how they feel. In that respect, Nvidia is leagues ahead. The 7970s stuttered constantly and so bad that I would have to limit my use to an hour at a time or it'd cause headaches, and 2hrs gave me a migraine. Now, some fanboys I'm sure are going to jump in and say that the new drivers fixed that, but I checked them out on a friend's rig and while unquestionably an improvement, they're still far from smooth.