Buffered (ECC) ram does error correction in memory which can prevent incorrect data from propagating to disk which is massively important in a enterprise environment, it'd be helpful on a media server to some extent and on an actual HTPC meh. ECC ram is barely more expensive than non-ECC so if your architecture can support it its worth it. As for Xeon CPUs, they're not magically different, there better binned chips that are rated to a higher standard as others have mentioned, like higher octane fuel.
Do you need this crap for a HTPC media server? No way. Would it be fun to work with? Hell yea! If I could run Handbrake on my media server I'd totally stick dual 4 core CPUs on there with ECC ram and SSD drives as buffers between rotating disks and the NICs. Oh and if I had the cash to toss around