Hi. A single 780 would in most cases, kill SLI 660ti's. If you can afford a 780, go for it, but i'd wait till Nvidia announces or releases their new series, the price should drop quite a bit. But, I'm running SLI 660ti's and they work well for me, I can run pretty much all games on ultra on a single monitor, and most games in high settings in Nvidia surround.
So, if you are going to be using one monitor, go with the SLI 600ti's. If you will be using more than one when gaming, go with the 780. The extra GB of VRAM will haul.
Good Luck!