It is pretty obvious that the new GeForce GTX 880 will be faster than the GeForce GTX 780, but what is interesting, is that it will be both cheaper, and more power efficient - something nobody can complain about.
The news is slowly floating out that the 20nm-based Maxwell-powered GeForce GTX 880 will be faster, cheaper, and more power efficient - but that is only one GPU in a massive lineup of cards. We should expect the new high-end Maxwell-based 20nm GPU late this year, or early 2015. NVIDIA could be forced to reveal 28nm-based Maxwell GPUs, but right now there's no threat to force their hand.
If NVIDIA do release the Maxwell-based GeForce GTX 880 on the cheap, faster than what we have with the GTX 780, or even the GTX 780 Ti, at around 200W power consumption, it would really put the pressure on AMD. This only ends up being great for consumers, though.