You might be able to tweak around that in the bios...does memtest and/or the BIOS memory count confirm the full amount of RAM ? Does memtest show any errors when checking.
Memtest passed with no errors. It displays 3967M of memory and under settings it displays RAM 417MHz (DDR835) CAS 5-5-5-18 DDR2 (128bits)
Often better memory support is added to later Bios updates..are you running the latest bios image for your systemboard ?
According to the Gigabyte product page last week I was out of date on F5 but have since flashed the BIOS and upgraded to F6 still with the same issues.
I still think it may be something relating to the 1.8v DIMM slots/2.1v memory.