Rambus DIMM has PC-800 for 400MHz CPU based such as Intel OR840 or i860 chipset DP Xeon, Intel D850MV Pentium-4, Intel 820 chipset boards. PC-1066 is for 533MHz CPU based such as Intel D850EMV2 Pentium-4 or similar design such as Dell 8250. Rambus is dual-channel memory design so a pair of same speed, same type ECC or NON-ECC DIMMs is recommended. Samsung MR16R1624DF0-CT9, 128MB, 1066-32P, PC1066 Non-ECC Rambus (New) $69.00 Kingston KVR1066X16-8/256, 256MB PC1066 Non-ECC Rambus (New) $149.00 Kingston KVR800X18-16/512, 512MB, 18-components PC800 ECC Rambus (sell as is) $169.00