Taken from:
http://www.microsoft.com/whdc/hwdev/platform/server/pae/PAEmem.mspx
Windows 2000 Memory Support. With Windows 2000 Professional and Server, the
maximum amount of memory that can be supported is 4096 MB (identical to
Windows NT 4.0, as described later in this section). However, Windows 2000
Advanced Server supports 8192 MB of physical RAM and Windows 2000 Datacenter
Server supports 32,768 MB of physical RAM using the PAE feature of the IA-32
processor family, beginning with Intel Pentium Pro and later.
Windows XP Professional and Windows Server 2003 Memory Support. The maximum
amount of memory that can be supported on Windows XP Professional and
Windows Server 2003 is also 4 GB. However, Windows Server 2003, Enterprise
Edition supports 32 GB of physical RAM and Windows Server 2003, Datacenter
Edition supports 64 GB of physical RAM using the PAE feature.
Windows 95/98/ME - maximum of 512MB
http://www.dewassoc.com/support/win98/win9x_512memory.htm
