Moore's law and the history of Nvidia's releases say that the gtx 880 will launch near Q1 2014 and run around double the performance of the gtx 680.
I would guess off the top of my head that a gtx 860 ti would perform 20% better than today's gtx 680 and be the best card to get before bottlenecking would start to hit you hard. If prices stay similar you'd be best off to but a 860 ti for $300 2 years from now. The 880 will just be too powerful.
But this is all speculation and nobody should be quoting me on this.
moores law has kinda tapered off recently though...
580vs680 the 680 is roughly 25-35% better
6970vs7970 the 7970 is roughly 25-40% better. that is no 100% better over 2 years...