PE 2600 & Win Server 2003 Ent. - How to enable PAE?

Also posted over in "General HW"...

Our PE2600 shows it can take max RAM of 6GB. We now have 6GB, but Windows Server 2003 Enterprise is not showing all RAM. I tried the /PAE switch on the end of the boot.ini string, but that did not work.

Is there something in the BIOS or other hardware that needs to be set first?