It really depends on how you game...
Are you playing AAA titles? Indy Games?
Are you Gaming at 1080p? 1440p? 4k?
For me, I have 2x GTX 770's I can play MOST games with the settings maxed out at 1080p.
I cannot game at 4k because the 770s only have 2GB of memory.
I will be upgrading when Nvidia launches their new graphics cards because I want to game at 4k, and I don't want to upgrade to the 900 series cards (not a big enough performance boost)
If you plan to stay at 1080p for a while, the 780 should still have a year or 2 of life in it (maybe even more if DX12 is all that it promises to be)