Wait for AMD's Hawaii GPU(s). The effect could be two-fold; cheaper GPU(s) that perform just as well (or better), and a price cut at Nvidia's side.
If you're at 1080p, honestly dual-GPU of any high-end GPUs currently available will be
mostly a waste except if you like to use AA in everything. For BF4, it remains to be seen, but I don't think it's going to be much more intensive than BF3. Turn off MSAA in BF3 and see how you hit a CPU bottleneck.
Oh, also, $650 is a ripoff for a GTX780. Especially with the HD7990 at $700. It eats it up. However, quadfiring that beast is just
asking for trouble. That's disregarding scaling issues that start showing up with 3 cards (Microstutter on AMD's side up till now, Nvidia's tri-SLi largely unexplored) and that are particularly a problem with 4 cards.