I have been looking at this again today and can see something curious/worrying...
On most of the boards which DO support the 7850, it shows a different BIOS version for the 7850, compared to say, the 7750...
This implies that the 7850 is slightly different and requires new BIOS... But for the GA-MA770-UD3, there arn't any new versions available (other than FB, which says nothing about the 7850)....
Surely they can't have released a board which doesn't support one of the popular AMD chips? Have they??
Jon