I have a PE 2650 (533 FSB BIOS A17) with (2) 512MB modules in Bank A.  I wanted to upgrade to 4GB, so I bought (4) Kingston 1GB PC2100 ECC 184 Pin DIMMs to put in Banks A and B.  Every combination of 2 or 4 of them with or without the original (2) 512MB modules results in a POST failure - no memory.  What gives?