Upgrading an hp z400 (six slots) - HP Support Community - 7156445
CMS 8GB (2X4GB) DDR3 10600 1333MHZ ECC Non Registered DIMM Memory Ram Upgrade Compatible with HP/Compaq® Workstation Z400 Workstation for Server Only - B80 at Amazon.com
HP Z400 Workstation Memory/RAM & SSD Upgrades | Kingston