Asus P7P55 WS. Tried all sorts of RAM (none on the QVL) and seem the have the same issue: Usable RAM is halved. Most of the time when I boot into the OS (Win7) it shows that 8 is installed but 4 is usable.
At the moment, I have 4 Crucial 4GB sticks installed in all four DIMM slots. I ran a WMIC memorychip and got the following: