Got it working again. Crisis aworded.
Reinstalled Windows and everything seems to be working. Did a couple of minutes in "Tomb Raider" and it works without glitches. (Kinda impressed by the card, didn't start the fans anytime during the short test)
Thanks to everyone who took time to help.
Lession leaned, don't touch a GPU bios again.
Don't know what happened, but it seems somehow the Gigabyte way of flashing the bios, .exe file downloaded from the cards support site on gigabyte.com did f. up Windows somehow.