Power supplies are recomended by processors manufacturers to ensure smooth operation of your computer. For example Intel recomend nothing less than 400W for Prescott based systems. This is not just to line the pockets of the 400W PSU makers! It is to make sure your system runs reliably!
If you try using a weaker PSU you will compremise the safety of your components. It might start by the computer constantly powering off and can end in damaging components which are sensitive to power flactuations.
Checkout the processors recomendations and follow them!
Bottom line (as you can see, I put it in the bottom

) - this is one experiment you do not need and is bound to fail!