What is Registered Memory and unbuffered memory and should I use it for my system?

Registered memory contains registers or buffers that take care of signal distribution to all the DRAM on a DIMM. Because the buffers take one full clock cycle (2 DDR clock cycles) to buffer the control and address signals Registered memory has a higher latency than unbuffered memory. The tradeoff is that buffering increases the reliability of high-speed data access which is critical for server operations.

As a general rule of thumb, Registered modules are only used in Servers and Mission Critical systems where reliability is crucial - this is particularly true in situations where a lot of large data is being moved. Regular home users and performance users will not get any benefits from Registered memory, and might actually see a slight decline due to the higher latency of the buffering.

Just how much the performance is degraded is hard to say. Latency in general won't result in massive changes in performance - we're talking about single digit percentage influences in the lower range here (2-5%)... but honestly, if you are building a performance system even a 3% drop in performance is not going to be acceptable to you.