A single GTX 780 offers up to 70% more performance than a GTX 580 according to NVIDIA so I suspect that you'd see pretty much the same framerates from one GTX 780 as you have for GTX 580, with some games performing better, some about the same and maybe even the odd one actually running worst (if it had, say, 80-90% scaling on your GTX 580 SLI). You can always add a second GTX 780 for SLI later if you feel you need it plus you'll be instantly doubling the amount of VRAM available for games from 1.5 GB to 3 GB, which is always nice. I'm not sure the Titan is particularly good value now the GTX 780 is out unless you need the 6 GB VRAM and/or double precision performance for non-gaming tasks.
If you can sell both your cards to cover most of the cost I would do it...especially if your running measly 1.5gb gtx 580s.
Hii in my case i have 2 580 amp edition and they givme alot of performance but to a high cost my light bill is very expensive just the 2 580 draw more than 500wats so i put one in the box and i play whit one until i need the other , so if i have the money i will buy a titan or the 690 same price.
A 580 doesn't draw much on idle so you could probably leave it in the PC and just disable SLi if you do not want to use the 2nd card. A GTX 780 will mostly match your GTX580 SLi performance and draw the same power as one GTX580. You get double the VRAM and quite higher memory bandwidth. I'd say it's worth it for gaming. Its price is somewhat elevated but it's a good upgrade nevertheless.
As stated, at idle 580's dont use much power otherwise I would have upgraded by now. The benchmarks I have seen show the 780 is about the same performance as SLI 580s. So if you have trouble getting SLI working in enough games or want less power use under load, the 780 will be a decent card. As Yasamoka said, amount of vram could sway you as well. A bit expensive maybe for the return, only you know its worth to you.
It will be worth it, from 500 to 700 is a nice jump. I had a 580 matrix for a little time, still beast today tbh.