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Computer/PC ATX Power Supply 
The power supply does exactly what it says, supplies power to computer and all it's components. Most power supplies come with all the necessary power connections for any typical computer and will fit in most standard cases. Computers these days are getting more and more powerful so it's important to get a power supply that has enough power. A power starved computer is apt to have problems so make sure you get a power supply with at least 300 watts of power. Just to make sure, you may wish to look up the model of your motherboard or processor on the company's website and they will recommend a certain wattage or even model of power supply.
There is the chance that the case you purchase will include a power supply so make sure you decide on the case first, and you may even be able to skip this step.
Cost
Power supplies by themselves can range from $35 to $150 (Average Cost: $65) mostly depending on the wattage.
Where to Buy
TigerDirect: Power Supplies