Registered RAM

"What is "registered SDRAM"?
Registered SDRAM is the SDRAM version of a buffered module. All memory must be buffered; it is just a question of where the buffering takes place. Buffer logic chips clean up the signals on a DRAM module. They don't boost the signal, they just provide a small buffer or cache where data can be held for 1 clock cycle prior to being written to the DRAM cells. This buffering only takes place on writes simply because a buffer is not needed when the memory is being readas the chipset can handle it as fast as it can get it. The term "registered" reflects the use of a register logic chip rather than a buffer logic chip, and the registered part is used because a regular buffer logic chip can't keep up with the speed of 100MHz SDRAM"

From:

http://www.crucial.com/support/faq.asp#D5

See also:

http://www.hardwaregroup.com/pipermail/hardware/Week-of- Mon-20000703/040411.html
http://forum.arstechnica.com/forum/ubb/Forum1/HTML/00685 3.html

Hope this helps.