This system supports triple channel mode when three identical modules are installed. Do not mix ECC Registered DIMMs with ECC Unbuffered DIMMs. To choose RDIMM or ECC UDIMM depends on which CPU and memory type you have. Intel Xeon 3500 series processor could not support ECC Registered DIMMs,TS8GJMA343H and TS4GJMA343H. Please check which CPU and memory you have before you buy the memory.