How to install 16GB of memory/ram into an HP Z400 Workstation - YouTube
A-Tech 4GB for HP HP Z400 Workstation (6 DIMM Slots) (1 x 4GB) PC3-12800 (DDR3-1600) ECC Unbuffered UDIMM 240-Pin 1Rx8 1.35V Server Memory RAM at Amazon.com
Arch Memory Replacement for HP A2Z50AA 8GB 240-Pin DDR3 1600 MHz ECC UDIMM RAM for Z400 Z420 Z620 Z820 at Amazon.com
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