How To Choose A Power Supply:
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It is a common misconception that high wattage PSUs are good, and the only characteristic you need for a PSU is watts watts watts. That idea is extraordinarily unrealistic and will only lead to premature failure, as theblackmages has already pointed out. I'll lead you through the steps of what you need to do to find a PSU that is right for you.

Step 1: Figuring out what you need.
This part is rather simple. Design your entire build first, then go here and fill in all the blanks. When you're done, you will see Surge Compensation and Capacitor Aging down at the bottom near your final output number. By reading what they said, it's a good idea to plan for some extra wattage for down the road when your PSU gets a bit worn out. I like to put on 25% Capacitor Aging and 10% Surge Compensation (For example, let's say your PC takes 258w @ peak load. Putting on 25% & 10% gives you 348w.) This tells you that you should plan for a 350w PSU so that it will continue to give reliable performance for years to come.

Step 2: What are you looking for in a PSU?
The next step in PSU selection is deciding what important factors you wish to have present in a PSU. Obviously you want a good one that isn't going to blow up, but do you need it to be quiet? Must you have flashy LED lights? What are these Modular PSUs and do I want one? There are a lot of things to consider. First of all, if you're looking for quiet, check out this Review from SPCR on quiet PSUs. They won't be cheap, but damn you'll be getting a nice one. PSUs with shiny lights are generally attempting to persuade you to purchase their products because they certainly don't speak for themselves. My reccomendation is to kick those junkers to the curb and go for the ones who give reliable performance. Modular PSUs are simply ones with a removable cable interface on the front of the PSU so that you only have to use the cables you need. Take a look at the Sunbeam NUUO for example. This is a nice solution if you are using a minimal amount of components, but if you plan on doing SLI, have two optical drives, and/or two or more hard drives, you're going to end up using all those cables anyway. Now, instead of having all the cables lumped in one corner of the PSU making them easier to hide, they're spread out all over the face of the PSU and are a bitch to conceal and group together. I tried the NUUO + SLI Ready config with multiple drives, and I would have much rather liked a non-modular PSU, so it isn't always a benefit.

Step 3: Amps, amps, amps.
Why are amps important in selecting a PSU? Simple; if your PSU cannot provide enough amps to your components, your PC will not function correctly because its components are trying to draw more amps than the PSU can provide, regardless of how many watts the PSU has available. A lot of the components in a PC are powered off the +12 rail, so this is the most important area to notice when looking at the specifications of a PSU. When looking at a PSU with multiple +12 rails, simply add all the +12 amps together, because they all work together anyway. I'm obviously not an Electrical Engineer, but that's the layman's version. How many amps do you need? Again, that depends on what hardware you have selected. A single graphics card PC with 2-4 hard drives and 2 optical drives will be happy with 20amps on the +12 rail. When you start getting into SLI or Crossfire gaming machines, expect to shell out some cash on a stable PSU. For a mid-range dual graphics card setup (2x6800GT, 2x7600GT, 2x x800xt, 2x x1600xt, 2x x1800gto), you will want at least 550w with about 28-30a total on the +12 rail. For high end dual graphics card setups (2x7900GT, 2x7900GTX, 2x x1800xt, 2x x1900xt(x)), look for something in the 600-700w range with 40-60a on the +12. Yes that may be overkill, but it's better to be safe rather than sorry.

Step 4: Picking the right PSU for you.
Quality is the #1 idea to keep in mind when selecting a PSU. Seasonic and PC Power & Cooling make the best PSUs on the market, so if you can afford their PSUs, you won't be disappointed from any of their offerings. Scythe, Seasonic, Antec, and Enermax NoiseTakers offer many low-noise solutions that are all of high quality, so you can't go wrong with their PSUs if you need to keep quiet. If quiet isn't so much a concern as power on the rails and lots of watts, OCZ, Fortron, and Enermax Liberty PSUs pack a lot of power and are priced conservatively. There may be some other good brands out there, but if you're looking for consistant quality, the brands in the Good list are the only way to go. Follow these simple steps, and you're guaranteed to find a good PSU that works for you!