I've just bought a dq45cb and put 3x 2048mb ram sticks into it. The bios (v0129, latest) sees all 6gb but the OS can only see 5gb of it. I've tried different linux kernels ranging from 2.6.24 to 2.6.38, Windows7, and various livecd distros and they all report the same thing.
The IGD is using 256mb presently but this doesn't account for the other 768mb lost to the aether. I've also tried with the IGD disabled and using a PEG card in the 16x slot, no difference. I've tested the ram in other boards and run memtest86 on them overnight.. they're all fine. I've checked the ram timings reported by the bios and they're fine too.
What's happened to that other 1gb??
As I want to use this machine for virtualization (I bought it because it has VT-d/iommu) this missing 1gb is quite a big constraint.