unbuffered memory = chipset deals directly with the memory. There is nothing between the chipset and the memory as they communicate. (most memory you'll buy is this kind. unbuffered is by far the most common)
buffered memory = a buffer is present to help the chipset cope with the large electrical load required when the system has a lot of memory.
registered memory = unbuffered modules that contain a register that delays all information transferred to the module by one clock cycle.
Buffered and registered memory's usually only found in servers and other computers where it's extremely important that data is properly handled. DDR and SDRAM modules can be registered or unbuffered; EDO and FPM modules can be buffered or unbuffered.
and i'm not criticizing your processor choice by any means, but just remember when chosing a processor. don't worry about the numbers (3200+, 2800+, 2500+, 2200+, etc.) look at the CORE (thoroughbred, barton, etc.). if you're going to pay $50.00 more for the same core but clocked a little faster, then that's not worth it.