I just purchased the LG 34UC97-S 3440x1440 and found for a lot of games that I play (Arma 3, Dragon Age: Inquisition, etc) with a single 780Ti HydroCopper, it's really starting to struggle even on medium graphics settings.
Personally, I'm looking to go for SLI 980's after I sell my GPU and do a little saving, as I feel the only way to justify this panel for gaming is to have the settings cranked right up to ultra. Though another 780Ti would definitely give you the performance you need to really push your games and keep a pretty solid 60fps!
I suppose it's down to your budget, if you can afford to, go SLI 980's and don't look back. It'll work out better in the long run, which will come in handy for the more graphically demanding games coming out this year and next.
If you want better price to performance and still have a more than respectable fps on most games with your settings cranked up, go SLI 970's. I'd imagine this route will carry you well for this year to come, though will probably start to suffer towards the end of the year with the more demanding Res you're using.
If you're on a budget and need a quick performance gain for a surprisingly cheap (if you're willing to shop around) amount, grab another 780Ti to SLI.
Just a personal opinion of course.
Good luck with whatever you decide to go with.