UPDATE:
SemiAccurate found an interesting chart, a comparison of GTX 770 to Radeon HD 7970 GHz Edition. Take it with a grain of salt, it’s from Charlie…
Apparently the GeForce GTX 680 can be modified into GTX 770 with a simple BIOS replacement.
All you need to do is flash a new .bin file to your ROM, then replace .inf driver file and you’re done. Despite the fact that the GTX 770 will actually use a newer iteration of the GK104 processor (.425), technically this will still be the same GPU.
It is unknown at this time, whether the new BIOS increases the voltage of the processor or memory.
The new BIOS changes the default clocks to 1059/1125 MHz for base and boost respectively. Notice, that the memory was also overclocked to 7GHz point. Therefore I wouldn’t recommend to try it, unless you have a Dual-BIOS feature.
Bear in mind though, you are basically just cheating your PC to recognize the new card as GTX 770. That is, until someone proves it actually bumps the voltage.
According to the leaker, the new BIOS will increase the performance by 5 to 7%. This could be an indicator about the GTX 770 performance. By the way, these are probably the exact specs of the card.
This is what happens in the .inf file:
NVIDIA_DEV.1180 = “NVIDIA GeForce GTX 680” ->
NVIDIA_DEV.1180 = “NVIDIA GeForce GTX 770”
Reddit (nvfan104):
The upcoming GTX 770 is a GTX 680 in disguise. NVIDIA has gently revised the GK104 architecture, mostly with process improvements, but the 770 is basically the same beast! Which is good for us enthusiasts. :)
By flashing the included BIOS (.bin) to your board, then dropping the included .inf file into the 320.00 driver package before installation… you too can have a GTX 770 weeks before it’s released! Please note it has to be a 320.xx driver, as support for the 700 Series was added in the DLLs around this time.
Remember, you are doing this at your own risk!
Source: Reddit
Many thanks to Alatar for a tip!