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?
Go for the ECC! The main difference between a desktop and a server is, that on a desktop you see a bit error a once. In most cases the user will not notice, it was a memory error. It looks like a typo in a document or email address a you can correct it direct. Most data on a server gets written to disk and has no consistency check. Imagine, you have a bit error in the payroll information!