When you hot-add Random Access Memory to Windows Server 2003 that has the Hot Add Memory feature enabled, the additional memory is unavailable and the System event log contains:

Event ID : 0
Event Type : Warning
Event Source : PNPMEM
Description : Failed to add memory range starting at 0x7F000000 for 0x300000 bytes. This operation exceeded the maximum memory capacity of the operating system or the server is not configured to support dynamic memory operations.

To workaround this problem behavior, you will have to restart your server so that the newly added memory becomes available.

NOTE: See Hot-Add Memory Support in Windows Server 2003