One 780, with the lowered price point, the 780 is going to be the best card you can get.
Pros
-500 Bucks (Base Price) Sli setups would cost more
-One card, less power, less noise, less heat, less stutter
-Almost power of a titan
-3 GB Memory (Base) Sweet spot
-Ability to SLi later and be better than all of these current setups