On the sticker on the side of the power supply, (not the case), it should state the maximum wattage...not peak necessarily even though that is important too.
It will look like
This.
Depending on the kind of cpu you have now you may not want to use your old HSF on a 2000+ XP CPU. They put off a fair amount of heat and if your HSF isnt rated high enough you could burn up the CPU.
The one I mentioned is retail so that means it comes with an AMD certified HeatSinkFan.