Un-registered is called "unbuffered" and that's what consumer-oriented motherboards use, both in the past and in the present. Registered, or "buffered," modules are used in servers and high-end workstation motherboards, to make a generalization.
So pick your Athlon64, pick your desired motherboard, and grab some normal unbuffered PC3200 memory modules to go with it, if you need a blanket suggestion.