ECC does run slower (usually) than non-ECC ram, and it does not work with non-ECC RAM. (ECC stands for Error Checking and Correction) I bought 256MB of ECC RAM about 8 years ago to add to the 256MB of RAM I already had in my system, but I had to replace the old RAM with the ECC RAM because I didn't research it enough to undertand the differences between the two (I figured that the slightly more expensive RAM was better, and while this is true in some respects, it wasn't for my applications.)