Question : Windows 2008 64bit system showing only 4GB usable...

I have a Dell R410 (PER410) Server running Windows 2008 64-Bit with 8GB of physical memory installed.

Even though the system is a 64-bit system (looking at the Computer properties, the "System type" section reads "64-bit Operating System") the system only shows that I have 4GB of usable memory. In the Computer properties, under "Installed memory (RAM)" it reads "8.00 GB (3.99 GB usable)." Why would a 64-bit OS not be able to use all 8GB? I have checked and double-check and I am indeed using a 64-bit OS. Can someone please shed some light on this?

Answer : Windows 2008 64bit system showing only 4GB usable...

Check the BIOS ... you likely have Memory Mirroring enabled.  That is an odd default on many configurations with the R410.  You can also check (and change) it in OpenManage Server Administrator (System, Main System Chassis, BIOS, Setup tab).