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You might be able to utilize all 6 GB on your server. It depends...
Windows 2000 is a 32-bit OS. All address pointers are 32 bits (max 4 GB virtual address space). But, it is more complicated than that. See (1).
In Windows 2000, 2 GB (or 3 GB if a switch is used) of the address space is reserved for the application to use. The rest is for the system. However, in certain versions of Windows 2000 (Advanced Server, Datacenter Server) the [u]system[/u] can support more [u]physical[/u] memory than 4 GB via Physical Address Extension (PAE) (2). This means that adding more physical memory to such a system will reduce paging to disk as the applications individually access their own 4 GB virtual address spaces that are mapped to physical memory pages.
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(1) Memory Support and Windows Operating Systems
http://www.microsoft.com/whdc/system/platform/server/PAE/PAEmem.mspx
(2) Operating Systems and PAE Support
http://www.microsoft.com/whdc/system/platform/server/PAE/pae_os.mspx