If it makes you feel any better we purchased almost 30 of those boards and all could not go over 4 gigs of ram.
Our issue is we use a program called unigraphics and it requires 8 gigs on board.....so all 30 had to go back, do not rack your brains over this...even Gigabyte has no solution and forum people will tell you you must be running 32 bit windows or your cpu is not seeded right, so you will get no help there either.
Again, the bios will never see more than 4 gig, so just dump the motherboards now.
I am out here looking for a soution to another problem with one of the 4 front office Gigatheybyte boards we have left, once we replace those we will be gigafree