Major con of multiple GPU's... microstuttering.
I "upgraded" to 3 way SLI not too long ago. My FPS went up noticibly, but the microstuttering drove me crazy and I went back to 2 GPUs.
You'll be happier with 2 780s, the 1000w power supply should be just fine.
Also, the 800 series is coming out in a few weeks. I wouldn't buy any GPUs before then. Just wait and see what they offer.