buffered/registered ram has a control chip built into the ram chip help the memory controller deal with large quantities of ram. you want to avoid buffered ram for a gaming computer (the extra controller chip adds latency). as stated it is for servers and workstations
avoiding fully buffered memory for a gaming machine I have heard before. however I have a dell server PC that has 16 gigs of ram dual Xeon 3.2ghz running win7 64 bit with a NVidiaGT express 16x 550TI with 3gigs of ddr5 graphics memory. and its a AWESOME gaming machine. I think servers that are max out and upgraded make very nice gaming machines, after all...the graphics cards does most of the work anyways. so having a server for a gaming computer isn't "ideal" but it does work nicely when you can max out a server with 32gigs of ram on a 64bit system with a nice graphic card
Buffered (also called Registered) RAM has additional hardware (a register) that sits between the memory and CPU, and will store data (buffering the data) before it's sent to the CPU. This is meant for reliability in systems that have lots of memory and lots of memory modules (think large servers), because in those systems more memory modules means more electrical demands, so buffering/registering the data reduces electrical load.
You don't really have to concern yourself with it if it's just some home machine, as it does tend to be more expensive (because it's more complex and intended for servers and machines of that class) and slightly slower (because of the memory buffering).
Also, some memory also is classified as ECC RAM, which has additional circuitry to determine if data has errors and correct them if so. It's also more expensive, and intended for reliability, and it's usually a paired feature with buffered/registered memory.