Thinking about redundant power supplies
The next time a salesman tells you that a server needs redundant power supplies for reliability, ask him if the server also has redundant CPU fans.
By now, most everyone understands that extended warranties on consumer electronics are just overpriced insurance (hence why the salescritter tries to get you to buy them, as they're great profit for the store). Redundant power supplies are getting to be a standard option in server configurations, and while they're not necessarily pointless I have to feel that a fair number of people are probably being sold ones they don't really need.
Buying a spare power supply is often a sensible thing. With redundant power supply designs, you are paying extra to have the spare plugged in and ready to go, instead of having to take some downtime to put it in. How much reliability do you actually gain, and how much is it worth?
Power supplies are not the only thing that can fail in servers; there's a lot of other moving parts, some of them under high stresses. Thus, having a redundant power supply may not actually be lowering the overall chance of system failure. (Or at least not lowering it by enough to change any important decimal points. The whole high availability field is an expensive minefield.)
Power supplies can fail in a number of ways; simply ceasing to supply power is the most benign one. With more explosive failures, there can be damage or problems outside the power supply itself, and your redundant power supply is unlikely to help. (It may even do harm, by keeping the machine running when an abrupt shutdown would better contain the damage.)
If total downtime is an issue (not just unplanned downtime), you need the power supplies to be hotswappable. Otherwise you'll have to take the machine down at some point to replace the failed power supply; the only question is when.
None of this is commodity PC hardware, which means that you're paying more in general. This is true even if you don't buy the redundant power supply; you're paying a markup on the base system for the mere possibility.
(So yes, this can mean that once you have to buy the base hardware for some other reason, you might as well get the redundant power supply; the extra cost may be entirely rolled into the base system price and the additional power supply itself not marked up.)