Hi,
The board has to support it. It has to be able to map the underlying memory to a higher address space, and the bios has to notify the OS where this is. Are you using a desktop board as opposed to a server board?
Anyone trying for 3>gb ram on an Asus p5b-vm should try their new bios 901 . 2007/07/18
Quote "P5B-VM BIOS 0901 Fix memory remapping function is not working properly issue."
Flashed mine and now 4 gb is mapped properly.
Shawn