2xSLI 770 > 1x 780, at about the same cost.
I keep saying it over and over -- the ONLY reason to get a 780 (or Titan) is if you plan to go SLI with multiple 780's (or Titans). Otherwise, a single 780 doesn't make sense from any angle.
As far as stepping up and then going 2xSLI 780's down the road, I'd say keep your money in your pocket and wait for the 800 series ~6 months from now [probably]. From what I gather, word on the street is that the performance jump in the next series ("Maxwell" I think they're calling it?) will be remarkable. Then again, with AMD's new release just around the corner, it's possible we'll see the price of the 780 drop to a more competitive level soon. Question is, how confident of that are you?
Asus RIVBE • i7 4930K @ 4.7ghz • 8gb Corsair Dominator Platinum 2133 CL8
2xSLI EVGA GTX 770 SC • Creative X-Fi Titanium • 2x 840 SSD + 1TB Seagate Hybrid
EVGA Supernova 1300W• Asus VG278H & nVidia 3d Vision
Phanteks Enthoo Primo w/ custom watercooling:
XSPC Raystorm (cpu & gpu), XSPC Photon 170, Swiftech D5 vario
Alphacool Monsta 360mm +6x NB e-loop, XT45 360mm +6x Corsair SP120