Updating bios with unsupported cpu?

The g680 should post fine. I use that board. bios file 0603 is required, which is one of the early versions. Mine came with the 4401. You need a usb flash drive for updating the bios. If your board works fine with it's current bios, I don't recommend updating it. Bad flashes can happen anytime. I've had it happen two or three times in twenty years of building. When it happens, asus has a bad flash recovery program, but it only works on some boards, and I've never tried it. In any case, if you don't flash it right away, save your old cpu so you can when you update to a faster one later.