The difference between "value" memory products and "performance" memory products is usually the timing. "value" memory requires more delay periods between memory accesses. "performance" memory requires less delay periods between accesses.

ECC, registered, parity, etc is not a difference. (Well, it is a difference, but not when you're comparing different memory for any one given system.) If your system requires registered memory with ECC, then that's what you have to buy for that system. Typically, server systems support registered memory with ECC support. I don't know of any typical laptop or desktop system that support ECC memory, so that difference doesn't play in the mix here.

For typical users going for a memory upgrade, I usually point them to crucial.com. They guarantee compatibility with your machine, and the memory has a lifetime warranty. It might cost a few dollars more than what you can find out there yourself if you know what you're doing. I doubt it will be more expensive than anything you can find at Circuit City.

-David