2666 will work, but will clock down to 2400 speed. The Z840 runs on 2133MHz or 2400MHz memory.Your current server configuration will determine the supported DIMM type and speed. When purchasing new memory it is best to purchase identical memory modules to what your system already has.Whereas the max RAM the HPE Z840 workstation can hold with ECC Registered is 512GB (16 x 32GB) of Registered DIMM (RDIMM) memory.The Maximum amount of RAM the HPE Z840 server can hold is 2TB (16 x 128GB) of Load Reduced server (LRDIMM) memory.This server will also support 32GB, 64GB or 128GB Load Reduced Modules (LRDIMM).The HPE Z840 will support 4GB, 8GB, 16GB or 32GB DDR4 Registered.The total number of DIMM slots for the HPE Z840 is 16.Cloud Ninjas' HPE Z840 Memory Upgrade Tips Different speeds can be mixed but the modules will function at the slowest speed installed. Along with the standard Registered RDIMM, the HPE Z840 also supports options for the Load Reduced LRDIMM. Each memory channel manages two DIMM slots. The HPE Z840 is a Dual Processor system with 8 DIMM slots per CPU.
