Registered RAM has the additional feature of checking data for errors, this is important for servers where downtime is very costly but to the average home user provides no real benefit.
Unbuffered RAM is usually cheaper (this may have changed) but does not provide error checking.
Using registered RAM requires that a motherboard have support for it, and in the case of AMD processors the CPU also has to have support for it.
Currently the only AMD CPUs that have Registered RAM support are the Opterons and some of the older FX-53 or FX-55 i dont remember, its one of those
Basically, AMD cpus with 940 pins.
Note however new amd cpus coming out will also have 940 pins but WILL NOT have support for registered memory - because AMD wants to make you cry.