Did you check out the link I sent? A single GTX 780 gets ~64FPS average, not going below 50FPS. A single GTX 780ti gets ~76FPS, not going below 60FPS.
Crossfire and SLI always introduce more issues than what a single card can offer. If you can, always stick with a single card. First of all, it consumes less power, secondly, it costs less, third, it performs on par or better, and fourth, you need not worry about driver issues.
If you think a single GTX 780ti doesn't offer the performance you need (It does) then you can go SLI. But, not before then, in my opinion.