Вот официальная инфа с сайта асус http://support.asus.com
This limitation applies to most chipsets & Windows XP 32-bit version operating system.
If you install Windows XP 32-bit version operating system, we recommend that you install less than 3GB of total memory. If more than 3GB memory is required for your system, then below two conditions must be met:
1. The memory controller which supports memory swap functionality is used. The latest chipsets like Intel 975X, 955X, Nvidia NF4 SLI Intel Edition, Nvidia NF4 SLI X16, and AMD K8 CPU architecture can support the memory swap function.
2. Windows XP Pro X64 Ed. (64-bit) or other OS which can address more than 4GB memory. You can check below URLs for reference:
http://dlsvr01.asus.com/pub/ASUS/mb/4GB_Rev1.pdf
http://www.intel.com/support/motherboards/server/sb/cs-016594.htm