4GB PC4-17000P-R ECC RDIMM Memory for HP Z440 Z640 Z840 Memory
16GB Memory for HP Z440 Z640 Z840 DDR4-2133 PC4-17000 RAM Upgrade
A-Tech 32GB Kit (2 x 16GB) for HP Z440 Workstation - DDR4 PC4-21300 2666Mhz ECC Registered RDIMM 2Rx4 - Server Specific Memory Ram (AT378360SRV-X2R2) at Amazon.com