Hardware Secrets has posted an article on FB-DIMM memory

FB-DIMM (Fully Buffered DIMM) is a memory module technology targeted to servers developed recently created in order to increase the memory speed and the maximum memory capacity of a server. In this tutorial we will explain what are the main features of FB-DIMM modules and the difference between DIMM and FB-DIMM modules.

How FB-DIMM Memories Work