There should be a setting in the BIOS for the RAM multiplier, 800MHz is x4 and 1066MHz is x5.33 (assuming you're running stock HT of 200MHz).
If you can't find it, when you get into the BIOS try pushing CTRL+F1, this enables many "hidden" options in Gigabyte BIOSes. Sorry I can't be more helpful, but I don't have the same motherboard as you.