ohh boy. I suggest you ship me your 680 and then get a 780. I'm not a fan of SLI unless you have a 144Hz monitor or 3 monitors. Otherwise, just get the 780.
I know the price of the 770 is fantastic and I think people will suggest that but I'm not one of them. The 770 4GB is around $460 and 780 3GB is $650. I think the decision could come down to if you need a 3 monitor system. If you don't then a 780 is better. If you want to get 4GB then SLI is better.