Registered vs unbuffered DIMMs

So I got myself an HP Z800 with 2x Xeon X5560's and am looking to upgrade the RAM from the stock 4 GB to 16(ish). The documentation states that the memory MUST be ECC, but can either be registered or unbuffered (but not both simultaneously).

What is the difference between registered and unbuffered memory?

See the link below for an executive overview.

http://en.wikipedia.org/wiki/Registered_memory

LOL Registered is just another word for buffered So the choice is buffered or unbuffered. Thanks beenthere, excellent link!!

Old thread, but since it turn up in Google i have to comment:
Buffered or registered, there _might_ be a difference. I do not know enough about old/new memory systems to say for sure:

A buffer is usually an un-clocked driver circuit. It will delay a signal by a fraction of a clock cycle
A register is a buffer that is controlled by a clock, and will create an exactly one clock delay.