AMD have nothing to fight of GTX 780.
They can just sit there and watch and run game bundle promos like they have done lately and hope people will buy their 2 year old 7970.
Tahiti (7950/7970) is too inefficient so they can`t make a bigger core based on it to match GTX 780. If they did, it would mean too much heat for the silicon and too much heat for the fan to remove. GTX 680 was 195W. GTX 780 have increased it to 250W.
7970 is already at 250W. A 7980 (example name) would be minimum 310W+ TDP on one single die. Good luck with that one.
Another thing is that AMD have ditched big dies. Nvidia master this, and have used it in the 400, 500 and now 700 series. Nvidia already said from the beginning that they were shocked by how little AMD could do on the 28nm and that they only had to release mid end GPU to match AMDs greatest. Turned out to be 100% true. GK104 matched Tahiti, GK110 which was Nvidia`s real high end is crushing Tahiti. Just like 500 series, the real high end was 500mm2 dies.
So AMD have nothing to do but try to sell 7990s (which came way too late since 690 have been here for an entire year) or sell 7970s with game bundles. In the meantime pray that their 20nm and/or new architecture is getting along nicely.
But I doubt it will be here until 2014. 6 months for Nvidia to cash in alone on the market with no competition. By the time AMD is releasing 20nm, Nvidia is soon after with Maxwell at 20nm.
Nvidia got AMD just where they want them. This generation, Nvidia won bigtime. Not just with desktops, mobile GPUs too (20% vs 80% market share)