I have a client with a PE2600. The BIOS recognises 2048 MB (4 x 512 MB ECC) of RAM but Windows Server 2000 only shows 256MB (less Video Memory) Otherwise the unit is fine. Seems a bit odd. I cannot find any documentation to solve the problem.
There was a similar post a few days ago, check in the sytem setup/Bios - F2 and look for a setting OS install mode. Make sure this is disabled as it limits ram to 256mb.