512MB PC3200 184 pin DIMM ECC Reg Question - What does the Reg after the ECC mean ?

DDR and SDRAM can be unbuffered or registered. EDO and FPM can be buffered or unbuffered. Buffered modules contain a buffer to help the chipset cope with the large electrical load required when the system has a lot of memory. Registered modules do not have a buffer but do contain a register that delays all information transferred to the module by one clock cycle. Buffered and registered modules are typically used only in servers and other mission-critical systems where it is extremely important that the data is properly handled.