I'm about to throw together a server for my cousin's small business because they are a bit short on cash and can't afford an IT admin ATM. It will be storing contacts and payroll information alongside a little miscellaneous data, AutoCAD files, and some encrypted business documents. Storage will be four three terabyte HDDs in raid 5. It won't be connected to too many computers: a receptionist's computer, three or four office computers, and a couple of workstations through a switch. The data that will be stored on this server will be kept for at least half a decade and really doesn't need to be lost. Would it be worth it to go opteron/xeon and thus ECC to help protect my data, or should I go with a regular desktop build and save a few bucks?
If you want reliability you buy a Proliant, as you're using large capacity SATA drives I would recommend RAID 6 as the rebuild times on RAID 5 SATA is nowhere near that of RAID 5 SAS. The company I work for does a lot of CAD work and very high res graphic designs and we have a 36TB SAN - less the capacity used for RAID 1 and RAID 5 LUNs so somewhere around 18-20TB usable space.
RAID is no substitution for backup so you also need to factor that in, you will likely need an LTO4 auto-changer, preferably fibre channel.
Non of this is cheap but you have to pay for reliability. Putting a Server OS on a desktop PC and expecting server performance and reliability is false economy in my opinion.