The amount of physical memory that can be safely allocated to a given product varies from system to system. While all products can manage the maximum limit that the particular Windows operating system platform permits, memory should be reserved for the operating system itself, and for the various processes (services and applications) that will be running. Additionally, when active, each product allocates a small amount of physical memory for overhead.

For systems with relatively small amounts of physical memory, up to 256 MB, the amount of available memory may be in the order of 30 to 50 percent or less. For medium sized systems, 320 MB to 2 GB, 60 to 70 percent of the physical memory may be available. On larger systems, above 2 GB, 80 to 90 percent or more of physical memory may be available for the product.

Each version of Windows (2000 Professional, Server, Advanced Server, Datacenter Server, XP, Server 2003, Web, Standard, Enterprise and Datacenter) limits the amount of physical memory recognized by the operation system. For example, Windows XP Professional 32-bit running on a machine with 8 GB of RAM will recognize only up to 4 GB. The remainder cannot be accessed.

The maximum amount of available physical memory at any moment cannot be predetermined precisely since it changes even as a system is active. Attempting to allocate the absolute maximum amount may ‘starve’ the system of memory resources, actually reducing its performance, and, in extreme conditions, could lead to a system crash.