It's always been my opinion that you should wait at least three generations of graphic cards before you consider the upgrade. From the 7xx series and since there was no 8xx series cards, I think you should wait until the 11xx series (the next generation) of cards before the upgrade would be considered "worth it".
Any upgrade from the 9xx series is worth it. You will be getting more than 50%+ performance boosted from the 780. I mean this is rough estimate but anything in the new series 1050, 1060, 1070 && the 1080's would be worth upgrading to without a doubt. Especially for future games, the 1070 will crush anything it get's thrown into.
It's always been my opinion that you should wait at least three generations of graphic cards before you consider the upgrade. From the 7xx series and since there was no 8xx series cards, I think you should wait until the 11xx series (the next generation) of cards before the upgrade would be considered "worth it".