I am guessing that your monitor is 60hz only as well.
And for that 680 should be more than enough, but I would still sell it and stick with 780 if you want to upgrade.
I have a GTX Titan and when you OC the 780 it can perform like a Titan. And you will have great time gaming at 60hz.
However if you can find a 680 on sale or something, that can be a good option too.
Me personally I try to have the best single GPU in the market and do not want the hassle of SLI. I would only go SLI if I absolutely have to. For 1080p 60hz you can just have a 780 and be very very happy for a year or 2.
Edit : SLI issues are getting ironed out a lot but there are still some horror stories out there.