Power Supply is indeed one of the most critical parts of your PC and it must be working without a hitch all the time - otherwise your PC will stop dead in its tracks.
Here are 5 Essential Things You Need to Know When Buying A Computer
Power Supply Compatibility & Connectors
What's the most important feature of a computer power supply? Well, its got to be its compatibility. Older motherboards will need a 20-pin ATX connector. Newer, more advanced motherboards will need the 24-pin ATX connector in a power supply.
If you look at a modern power supply in a PC, you will see it has many of those octopus-like connectors. Those are really 12V Molex connectors and they hook up to peripherals like hard drives, CD and DVD-ROM drives and PC case fans. You need to make sure the number of Molex connectors in your power supply is enough to meet the number of components in the PC.
SLI and Crossfire Technology
What does SLI do? It allows you to combine the processing power and memory of two Nvidia video cards. There is an equivalent term called 'CrossFire' if you're using ATI video cards.
Think about that for a moment - we're talking about insane performance. If you intend to go the SLI path, then your power should be SLI Certified (if you are using Nvidia video cards) or ATI CrossFire Ready (if you are running ATI video).
Overrated Peak Wattage Numbers
One thing you should always know about purchasing power supplies is that the peak wattage is usually overstated. If you building a system, going for anything less than 400W is not a good idea.
Size of the Power Supply
Another factor to consider is the size of the power supply unit. Most of the power supplies out there conform to the ATX specification. Make sure that your case is also in the ATX form factor if you are buying such a power supply.