You probably noticed that many cases on the market today come with their own power supply.  For most uses these power supplies will be adequate.  For gamers, power users, and server applications these usually do not provide the power necessary for the applications that they will use.  When looking at the power supplies there are several factors to look at:

• wattage
• size (very important as some sizes may not fit in the case you have selected)
• shape (also important when attaching to a case)
• noise rating (yes some of them can sound like a plane landing)
• form factor (20 pin and 24 pin for connection to the motherboard most new models come with the 24 pin)

For basic computer configurations I recommend a power supply of at least 300w as they will support the devices that you will use in every day computing.  For power users I would recommend a power supply of at least 450w.  And for gamers I would recommend a power supply of not less than 500w.  While these seem like very small jumps in power the price difference can be staggering the highest price for a pc power supply at this post was $579.00 for a 1000w gaming power supply and the lowest price was $11.99 for a 300w basic power supply.   For servers you need to find dual redundant hot swappable power supplies so you can change it while your computer is running if necessary.