If you want a future proof machine, you should travel to the future and buy a machine then. That would be cheaper too.
But seriously: If you want a new pc before next spring, go for the 700 series. If you can wait, it always pays off to wait. There is always better hardware on the horizon. Nobody that does not work at Nvidia knows how much better the next chip will be, and it seems like the next release is sometime 2015, so it is impossible to answer this correctly. The 780 and 780ti are very good cards, and will probably keep your pc good for several years.