^That is true, and something to consider. If you decide to go with one GPU, it may be a good idea to see what AMD is doing with the 8970s. Both GTX780s and 8970s should have a TDP around 250W, but one could be better, or worse for folding, only time will tell on that front, but one will cost less.
How does the 770(680) compare to the 7950 and 7970? I'm thinking if 2 x 7950s come close to 2 x 7970s I'll just go with the cheaper solution.