The easy answer is that almost any power supply unit will work just fine in your computer, especially if your computer is typical rather than high-end. However, some considerations can help you to make a more informed purchase. These include power efficiency, connector adequacy, hardware compatibility, noise performance and cost effectiveness.

Power

The most important aspect of a power supply unit (PSU) is, not surprisingly, its capability to supply power. Specifically, it must be able to safely and efficiently supply whatever power your components require, including both continuous power demands and demand spikes. Most units are rated between 400 to 1200 watts, but these ratings are unreliable. As of 2009, ratings are self-reported by manufacturers and are often misleading or outright fake. Some ratings conflate continuous power capacity with peak capacity, the latter being a much higher number but also an unsustainable one. Some ratings advertise total power capacity rather than capacity on just the 12-volt lines, even though the 12-volt lines supply almost all of the components. And some ratings assume an unrealistically low ambient temperature, which is unrepresentative of the inside of a computer tower. Without doing research, you would have no way of knowing whether the power rating on a PSU is accurate. One way to get around this problem is simply to purchase a PSU that is much stronger than you need. However, it wastes power to run a small load on a big machine, and, on top of that, PSUs are designed for optimal energy efficiency near the 60 percent capacity range. A better solution is to only consider buying PSUs that give specific ratings. Look for units that are rated for "continuous" power, which list capacity specifically on the 12-volt lines and give efficiency ratings at a realistic ambient temperature of 100 degrees Fahrenheit or greater. An overall rule of thumb is to buy a PSU that exceeds 160 percent of your hardware's rated demands or 110 percent of your hardware's peak power demands---whichever is higher. This includes not only present hardware, but anything else you are likely to add in the future. When you're calculating your system's needs, don't include components that plug into a power strip rather than the PSU, such as speakers and monitor.

Connectors

You will need to buy a PSU with enough connectors for all your components. There are several types of connectors. Most of your components will require a SATA connector, a peripheral connector or one of several versions of PCI-E connectors. Check to see which of your components require which connector, and add them up to determine the minimum number of each type of connector you will need from a new PSU.

Compatibility

These days PSUs are highly standardized, leaving few ways for a unit to be physically incompatible. Most importantly, make sure your PSU matches the form factor of your computer tower. Also, if you run high-end graphics that use multiple video cards, make sure any PSU you consider buying is certified for SLI or Crossfire.

Noise

If noise is a big concern for you, buy a PSU with a wide-diameter fan, responsive fan control and a large surface cooling area for faster heat dissipation. Some PSUs also come with rubber padding that muffles vibrations, further reducing noisiness.

Cost Effectiveness

Rarely do you need to spend money on top-of-the-line models. Middle-of-the-road get the job done just as well. Avoid cheap units, though, especially if you are buying a higher-wattage PSU.

Source:

HelpWithPCs.com: Power Supply Unit Basics

SilentPCReview.com: Power Supply Fundamentals