a decent 250watt ps will be fine for a 1.33ghz system if you don't load it down with a lot of stuff. basically, if you have that system, 1 7200rpm hard drive, 2 cd-rom drives, a few fans, and no more than a geforce2 gts you should be ok. if you want a more power hungry video car or many more devices or fans you are probably pushing your luck. i have a fairly robust system with a 300watt ps that a 250watt couldn't handle...it's better to be safe than sorry