Due to chipset limitations on the Intel Xeon 34xx processors the following rules apply: Unbuffered ECC DIMMs are limited to TWO Dual Channel Banks (4 DIMMs) per processor When two Quad Rank modules are installed per bank, the processor will automatically clock them down to 800MHz. This system supports either Registered ECC or Unbuffered ECC memory. Mixing Registered ECC and Unbuffered ECC modules is not supported within this system. Only Xeon CPU can use RDIMM, other CPUs cannot support RDIMM. To choose RDIMM or ECC UDIMM depends on which CPU and memory type you have. Please check which CPU and memory you have before you buy the memory.