Hello spilot
You cannot 'register" or "buffer" memory; it's the type of memory hardware spec that you purchase. Typically, unless you're running a server or some type of highly mission-critical system, you won't require 'registered" memory. In today's systems, where 1G of RAM is getting to be more the norm, I have a feeling Sandra needs to catch up to the times regarding those specific "warnings".
Registered Memory: A memory module that contains registers that hold the data for one clock cycle before it is moved on to the motherboard. This process increases the reliability of high-speed data access. Registered memory modules are typically used only in servers and other mission-critical systems where it is extremely important that the data is properly handled.
Here's a short (albeit older) article about
unbuffered, registered and ECC memory.