If you think the GTX 780 is an upgrade from your 680's, you are terribly wrong.
Your GTX 680's will significantly outperform the 780 is any game that scales with multiple cards in SLI (EVERY big title)
the GTX 770 is actually the same as the 680.
It just uses an updated version of GPU Boost and is equiped with faster memory chips.
You can flash your GTX 680 into a GTX 770 and get GPU Boost 2.
As for the memory chips, they don't really have a big impact on performance.