Recently I received a question regarding how much memory (RAM) is supported in Windows Home Server.
The answer is fairly straightforward. Microsoft built WHS upon the Windows Server 2003 32-bit kernal. Practically speaking, this means that WHS does not have support for RAM amounts greater than 4 gigabytes (GB) thanks to Physical Address Extension (PAE).
With that being said, the actual amount of RAM that can be utilized can be significantly less. If your WHS uses either an integrated or discrete graphics solution that has onboard memory, that memory counts against the 4GB upper limit of the 32-bit OS. While this would be illogical, if someone were to use a 2GB AMD/ATI Radeon 4870×2 or a 1.8GB NVIDIA GTX 295 GPU in their WHS, the effective amount of RAM that the operating system could use would be approximately 2GB and 2.2GB respectively. Again, using a heavy duty graphics card is more or less pointless in a WHS, unless you are running WHS in a Virtual Machine VM and have some other use for the card. RAID controllers, and various caches can eat up some of this 4GB limit also. There is a good chance that users with discreet GPUs, one or more RAID controllers, and other components with onboard memory will be able to utilize significantly less than 4GB of ram. Oftentimes 2.75GB – 3.25GB usable of the 4GB is not uncommon.
When picking hardware, Intel LGA-775 and LGA-1156 platforms can potentially get by with 2GB of memory (2x 1GB DDR2 sticks for dual-channel operation) and be close to the maximum RAM that can be addressed by the WHS 32-bit operating system. LGA-1366 systems will probably be best off with 3x 1GB DDR3 DIMMs. There is little point installing more RAM than the operating system can address because having unused RAM idling in the server will increase power consumption by a small amount and generate additional heat. Also, omitting extra DRAM sticks will usually allow for greater airflow around the already installed sticks.
Bottom line, WHS cannot address more than 4GB of memory (RAM) due to PAE. That number is reduced by things like GPU memory, RAID controller memory, caches on other components, and etc. There is little point in overbuilding the WHS to have more RAM than it can utilize.