Depending on the motherboard, if you used all of the DIMM slots, some M/B would require registered memory, especially if you were using the 512 MB memory modules. If you use ECC DIMMS you will take a slight performance hit 2-3%. This is neglegable unless you are OC'ing your system. High end systems, especially servers usually require registered memory with ECC. I use registered memory with ECC on my new system MSI K7T266 Pro2 RU motherboard and have noticed the memory scores are kind low compare to other people are reporting. My system is very stable and have not have any problems.