Registered means that they fit a registered DIMM or RIMM slot. Buffered means that the memory module has an additional chip to "manage" the memory, well delay it by at least one cycle. That is done to help addressing issues that arise when the memory is under heavy workloads.(ie servers and data warehousing) Normally a registered module will be buffered, whoever it might not have a buffer, but I have not seen one lately.
1. Are you building a server? If no, you do not need registered memory.
2. Are you looking to load your memory as in the case of data warehousing? If no, you do not need it.
3. Are you looking for memory accuracy and said no to the questions above? You can purchase ECC memory. ECC stands for Error Correction Code. If you have one bit errors in your memory it can correct the errors. This is also something servers, etc., use but everyday people also can use to minimize freezes, hang-ups, reboots, and errors.
I hope that this has helped...
<b>"I'll have a steak sandwich and a steak sandwich." - Fletch</b>