How to choose the correct pc fan?
The first thing is to match the size, because if you don't modification to the chassis will be necessary.

Then you'll probably want to match the connector (which will also match the voltage)  

The fan needs as much current as the channel it sources current from on the power supply (example, if the fan uses a molex connector with 12V @ 0.3A then the power supply on that channel needs to be able to source 0.3A. Generally this isn't a problem since most power supply channels can source 2 Amps or more)

If you get a fan that has the same voltage, and less or equal current you can't go wrong.