No experience with 580s (SLI or single), but last I checked (and it was a long time ago) a Titan (which is practically the same as an overclocked 780 in games) will give a 100% boost in FPS over a 580. While that sounds like the same performance as SLI 580s, its not. Scaling is never 100%, so you will definitely see a boost in FPS and more so when you overclock it to the ~1200MHz range (which I should mention is very much achievable with 780s, unless you get too unlucky with the card).
Also, single cards feel smoother (this is subjective but I think that most will agree) than two or more cards. So your gameplay experience will definitely be much better with a 780.
Then there's the matter of VRAM. With 3GB of VRAM on the 780, you will be much better prepared for the next-gen console ports which will most likely need much more VRAM than what we are used to seeing. This is a reason why I would recommend against 770 SLI like others mentioned.
Single cards are much more hassle-free than SLI setups. Less heat, power consumption, micro-stuttering, requirement of space in case are some of the major points why I'd recommend a 780 over the 580 SLI, and even 770 SLI for that matter. Because if you are gaming at 1080p, you dont really need much more than an overclocked 780 to get decently high frame rates unless you plan on going crazy with AA. I speak from experience with the Titan.
I didn't quite get if you can get $200 for each of your 580s or for the both of them together, but in any case, go get the 780 and enjoy!