Lets see what the future holds for single GPUs
Sapphire will later announce their 1335 Mhz 7970 flagship with their Atomic Series, which will come in air as well as liquid cooled variants.
This card will be clocked +45% higher than a standard 7970 (925 Mhz), giving roughly 40-45% higher performance. It will be intresting to see how high these clocks can go with further over volting and cooling, though I dont expect it to easily clock higher than 1400 mhz.
In August 2012, Nvidia will be unveiling their GK110 (GTX 685) single GPU flagship. According to the specs, it will cost $650 USD, and will sport roughly 50% more cores, and ~46% higher computing power, and ~50% more Gaming performance when compared to the GTX 680.
Both these cards will be beasts, however I still believe Nvidia's singlle card flagship will come out with a slight advantage. By August, AMD will already have released their 7990 dual card solution (watch in the next few weeks to come), which will still out perform both of these single cards by over 50%, despite its meager 850 Mhz clock. Nvidia's dual card solution will spot 2 x GK104 cards, so it will be neck and neck with the 7990 in performance. It is expected to be released in May of 2012. The auguest release date is not official, and this card could end up being the GTX 780 in late 2012. This would be the largest bump in performance Nvidia will experience between generations of their single card flagship.
Only time will tell how things unfold!