From MSDN:

On 32-bit versions of Windows, PAE is disabled by default. You must use the /pae boot parameter to enable PAE.

However, PAE is automatically enabled when the computer is configured for hot-add memory devices in memory ranges beyond the 4 GB region, as defined by the Static Resource Affinity Table (SRAT). Hot-add memory supports memory devices that you can add without rebooting or turning off the computer. In this case, because PAE must be enabled when the system starts, it is enabled automatically so that the system can immediately address extended memory that is added between restarts. Hot-add memory is supported only on Windows Server 2003, Enterprise Edition and Windows Server 2003, Datacenter Edition, and only on computers with an ACPI BIOS, an x86 processor, and specialized hardware.

Not sure about the /3gb switch - we're running Windows Server 2003 Enterprise Edition.