Gaming at 5760x1080:
You should monitor with FRAPS to see what frame rates you're getting. If you fall BELOW 60FPS (VSYNC'd) your options are:
1) another 780 for 2xSLI, or
2) adjust QUALITY SETTINGS to maintain 60FPS (recommend Adaptive VSYNC if it falls below 60FPS periodically)
3) run WITHOUT VSYNC (screen tearing)
Personally, I prefer a single, large monitor (27", 2560x1440) and game usually at 1920x1080 as gaming at 2560x1440 in most games causes a 30% hit with almost NO difference in quality. Exceptions are games with SMALL TEXT or HUD elements like Diablo 3, Starcraft 2, Torchlight etc. but these I can run at 60FPS no problem at 2560x1440.
That's with a GTX680.
Occulus Rift:
I'm eventually getting one of these, but likely in two or three years when resolutions are much higher and other issues have been sorted. Not a big deal for you now, but worth mentioning.
Summary:
Other than getting another identical 780 for 2xSLI, I'm not sure what else to say. You'd have to determine if that's worth it to you. I guess it partially depends on whether you can maintain 60FPS VSYNC'd with the same, or ALMOST the same graphical quality, or desire 3D.
Another option is to REPLACE your middle screen with a 27", 2560x1440 version and game solely with that. The pro is it's a larger, higher resolution screen. The CON is obviously not having three screens (which I dislike anyway due to the BEZEL SPLIT). The COST is actually about the SAME as a GTX780 but you wouldn't a second one (unless gaming in 3D at full quality etc.)
Your probably set on gaming with THREE monitors but I thought I'd mention the options.