I got the same problem on my DX79SR with Gigabyte GeForce 780GTX.
Unfortunately there are no BIOS updates from Gigabyte yet.
But there is a work-around:
1. Put both GF780 card and my old video card on the motherboard
2. Physically connect the monitor cable to the old video card and in BIOS set it to boot from old video card. (This will allow the computer to boot into Windows)
3. Download and install the latest NVidia driver. (and reboot if necessary)
4. While Windows is active, disconnect the monitor cable from the old video card and connect to the new video card.
5. One more thing, natively PCI-E 3.0 is not enabled, so you may want to enable it using this file, provided by NVidia. http://nvidia.custhelp.com/app/answers/detail/a_id/3135/~/geforce-600-series-gen3-support-on-x79-platform