[Butcher]

If it's a p3 you should be able to do something with pelts and air cooling. Most people go for athlons which you need to use water to cool a pelt on hence the reaction you got. If you don't say these things people assume... Something like an 50-80W pelt should give reasonable results with a p3, they put out about 20W of heat at 733 iirc.

As for the 650W being powerful enough, 650W is easily enough in and of itself, but a 650W psu doesn;t put out all the power at 12V, most of it is 3.3V and 5V, which aren't much use for pelts. The specs for your PSU can be found on the side or top of the unit.
The 650W model in fact has a 12V output of 24A max according to their website. This equates to 288W, your drives and such will suck up some of that - running a 156W peltier off your 12V rail is pushing it a bit unless you have a minimal amount of drives. For a smaller unit suitable for a p3 the 650 is fine though.