powersource performance

I have a pc box with a powersource that supplies rated 250 watts. I originally used it with an AMD k6III 400 processor. I am now using it with an AMD athlon 950. I really enjoy the upgrade, haha, non the less, I was planning on buying like about a 1.4ghz AMD processor but have been told that I need a 300 watt powersource for anything over 1.0ghz. does anyone have an opinion on this issue?