When shopping for a new power supply you should consider two factors: first the number of watts the power supply produces, and second the number of power cables that come with the power supply.
Watts: Power supplies range from 150-400 watts, and the number of power cables vary. Obviously, the more watts and the more power cables, the better. But, don't overbuy on the watts side if your PC doesn't need it. You will just be spending your money needlessly.
Power cables: The more power cables that come with your power supply, the more peripheral devices you can connect. Make sure the cables provide the large and small power connectors.
Size: Power supplies come in many sizes. When shopping for a new one, you may want to bring your old power supply along. That way you can make sure the new one will fit in your system unit.