Recently we added some 36GB to our Windows 2003 servers, but for some reasons it could only see 16GB when it reboot. We found not issues with the hardware since the agent reports the 36GB available.
Apparently, this is a known issue if you configure /3GB and /PAE on a Windows 2003 32-bit OS. This known fact is hidden somewhere in this MSKB, which I thought could have been more prominent! -> http://support.microsoft.com/kb/283037
The option is of course to remove the /3GB switch which most applications don’t really need, unless you are talking Exchange servers.