So I have an older Intel S5000XVN motherboard, here are the specs

CPU Arch : 2 CPU - 2 Cores - 2 Threads
CPU PSN : Intel Xeon CPU 5160 @ 3.00GHz
CPU EXT : MMX, SSE (1, 2, 3, 3S), EM64T, VT-x
CPUID : 6.F.6 / Extended : 6.F
CPU Cache : L1 : 2 x 32 / 2 x 32 KB - L2 : 4096 KB
Core : Woodcrest (65 nm) / Stepping : B2
Freq : 2985.21 MHz (331.69 * 9)

MB Brand : Intel
MB Model : S5000XVN
NB : Intel 5000X rev 31
SB : Intel 6321ESB rev 09

RAM : 16384 MB FB-DDR2 
RAM Speed : 331.7 MHz (1:1) @ N/A
Slot 1 : 2048MB (5300)
Slot 1 Manufacturer : Kingston
Slot 2 : 2048MB (5300)
Slot 2 Manufacturer : Qimonda
Slot 3 : 2048MB (5300)
Slot 3 Manufacturer : Kingston
Slot 4 : 2048MB (5300)
Slot 4 Manufacturer : Qimonda
Slot 5 : 2048MB (5300)
Slot 5 Manufacturer : Kingston
Slot 6 : 2048MB (5300)
Slot 6 Manufacturer : Qimonda

As you can clearly see, CPU-Z says I have 16GB of PC2-5300 RAM installed.

For some reason in both BIOS and in Windows the maximum usable RAM is 12GB instead of 16GB. I have a dedicated video card connnected, so it can't be stealing RAM to use for the GPU (the S5000XVN does not have any onboard video).

I am running Windows Server 2008 R2 x64 as the primary OS - so there should not be a memory limitation imposed on me from the OS.

Has anyone experienced this before? Any ideas on how I can actually use all 16GB of RAM?

I plan on using this machine to use for a Hyper-V server and have x2 Quad Core Xeon CPUs on the way as I write this post.