The GTX780 will always be more capable than a GTX770.
But, I think you are asking about how long a GTX780 will perform well as new games come out.
My guess, and it can only be a guess, is that a GTX780 will game very well so long as you are using a single monitor at 2560 x 1600 or less. If you contemplate triple monitor gaming or a 4k monitor, then all bets are off. I do not think game developers want to produce games that require such high end graphics that it limits their market.