4GB 2 X 2GB PC2-5300F 667MHz 240 pin DDR2 SDRAM ECC Fully Buffered FB DIMM Server Low Power Memory for HP Workstation XW8600 (PARTS-QUICK BRAND). Equivalent to HP Part Numbers: 397413-B21, 442823-B21,
IBM 4 GB DDR2 SDRAM 4 GB 667 MHz DDR2-667/PC2-5300 ECC DDR2 SDRAM Memory Module PW2821 at Amazon.com
Best Buy: Kingston Technology 4GB DDR2 SDRAM Memory Module KTA-MB667K2/4G