Gigabyte is a realiable mobo, as is asus, my only beef with gigabyte was on most of their boards about a year or 2 ago they were leaving the SATA connections off. They were available, but you had to solder them yourself... if you've ever seen me solder you'll know why i have a problem with this, lol. Even with this flaw they are still the best company i've ever dealt with.
My friend is running a similar system with the new AMD asus board (which he had to buy after a surge destroyed his gigabyte) and his computer is running atleast as good as it was before. When i look to upgrade again both of these companies will be definate contenders.