If you are building a new computer, or find yourself needing to buy a new power supply for your existing machine, my best advice is to not "cheap out." The power supply is a crucial part of your computer and a lot of the time the ones they install into Acer and HP desktops seem to be built to be just enough for what's in that case when you bought it and anything more is a strain on it.
Let's talk about tolerance for a sec. Even though the speed limit is 100 kms/hr., most cars on the road these days show on their speedometers that they are registered to do almost twice that speed. Why? Why not make cars so they can only do the speed limit and no more? Because then they would be underpowered. You wouldn't want a car that was on the teetering edge of exploding when you're doing the speed limit.
The point here is to say that you should not buy a power supply that is just enough unless you don't care about your computer. A weak power supply can (and most likely will) cause other components in your machine to over work and become weaker as a result.
If a new power supply, locally, is under say, $50 - $60 range, and promises 500 watts and up, it's probably a cheap power supply.
Feel the weight. If it's really light (like under a pound), stay away. A good rule of thumb is that that the heavier a power supply, the better the quality.