Power supplies are a bag of worms, and I mean this in more ways than one. You have a mess of wires, and you have a mess of specs, you have a mess of power requirements, and yet out of all to this comes a smooth running computer.
A computer power supply takes 120 volts AC that you get from the power outlet in your home into its black box and out comes DC voltages +12v, +5v, and +3.3 volts that the computer uses to power its components. The power supply is really a power converter. Since it has these different voltages being output to different components, each of which has its own standard power plug, what you have out the back of the power supply is a mess of wires with different plugs attached to the wires. And it is a mess.
Power supplies are described mainly by a wattage rating, for example 450w. This stands for 450 watts. The higher the watts the more power or current going to the components in your computer. Well that's simple, Not. It turns out the different manufacturers have different ways to determine the wattage of their power supplies depending on how they state their specs. You'll see specs calling out peak power, continuous power, and multiple rails with current limits all of which can be twisted depending on how their stated or not stated. The fact is there is no standard for describing and specifying power supplies, and as a result there are some pretty shoddy power supplies out there. The specs are a mess.
In fact it is so bad that I need to caution you about taking a Dell or HP power supply out of your old computer and using it in your new computer. Dell and HP went so far as to make power supplies with standard internal connectors with different voltages on different pins. Plugging this into a modern motherboard and components could instantly damage those components.
How much power do you need? Unfortunately, most component and graphic card companies do not state the power needed in their specs. The idea that you can add up all your power requirements to get the total amount of power needed is also a mess.
Let's approach this from a different perspective and see is we can pick out a decent power supply for your computer. Most computers without a lot of components run on about 350 watts. Now having said that, I would not purchase a 350w power supply and think your all set. I don't think they make one anyway. You always want to go over the watts you'll need. So for the average system, nothing fancy, I recommend a 500 watt power supply. Now if you start adding multiple disk drives, hot graphic cards, or double graphic cards, then you are going to need a lot more power, at least a 750w power supply. Those basically are the two main sizes you'll see for the home computer. Disk drives not so much, but as soon as you go hot graphic card, which now require their own power connector, I would move to the 750w power supply.
Next you want a power supply that has the letters ATX in the spec. What this means is it is compatible with an ATX motherboard. It has the proper plugs and proper power to the pins for an ATX motherboard and components. When you turn the computer off or go to standby power with the software, the power supply to do the same. All most all power supplies have this, but I thought I'd mention it anyway. Some power supplies also state they are ready for Crossfire or SLI. This refers to running two separate graphic cards in your computer, see my previous graphic cards posting for more on this.
Get a power supply with an on-off switch on the back. This allows you to absolutely kill all power to the computer. Power supplies without the switch means you have to pull the power plug to get the same effect. You want to kill power when your working on the inside of the computer, in lightning storms, and when your computer hangs and you can't shut it down.
Some power supplies are modular, which means that you are given a separate bag of internal power cables and plug them into the back of the power supply only if needed. This cuts down considerably on the wires inside the box and improves overall air flow, reducing heat. It use to be modular cabling led to problems if the cables became loose, most modular cables today have positive latching to make sure that doesn't happen, but you will not find this anywhere in the specs. Non-modular power supplies come with all the cables coming out of the back of the power supply. Cables you don't use have to be pushed into corners of the case. I recommend a good modular power supply.
Power supplies come in a standard size of 5.91" width, 3.39" height, and 5.52" depth. And as a result you will not see dimensions stated with your power supply specs. This size fits perfectly into your computer case which is built to include a power supply with those dimensions, easy. Well not so fast. The first two dimensions are the closest thing we'll find to a standard in power supplies, but the last one depth you have to watch out for. On your bigger power supplies, the 750w version, some manufacturers have elongated the case to about a 7.48" depth. They'll sneak this into their spec by stating dimensions, and not say anything else. I do not recommend elongated power supplies. They may or may not run a tad cooler, but they can create problems in fitting into your case as you start to wire up your computer. Stick with the standard size.
Which bring us to heat, power conversion takes energy which is expelled in the form of heat. Power supplies get hot. Every power supply includes a fan that cools the power supply. The problem is noise. I recommend the power supplies with the bigger fans rather than the smaller fans. Smaller fans run faster and make more noise. Curiously, fans always are mentioned in the spec. Stay away from the 80mm fan. Some manufacturers will stress quiet in their fan spec. That's goodness.
In picking out a power supply, look at the reviews and awards. As I said, because of all the messes above, there are some shoddy power supplies out there that will crap out on you after a very short use, and have flaky and unstable voltages. Unstable voltages can cause all sorts of problems in your computer that you will have trouble tracing back to your unstable power supply. Currently, I am using Corsair modular power supplies. Look for the word "modular" in the spec as they make both kinds of power supplies. At this writing, they make a 520w, 620w, 650w, 750w, and 850w modular power supply. I have used both the 520w and 750w with good results.