My vote is for at least a good-quality 300W, or maybe more to give you some cushion for upgrading in the future. They say the cheapest way to buy, is to buy the right thing the first time.
If you have a board that takes Durons over 1GHz, you might check to see what AthlonXPs it supports. Since AthlonXPs have a faster front-side bus speed and more level-2 cache, they might be a better choice for just a little more money.
If you want a particular suggestion... before AMD stopped publishing their recommended list with specific power supplies listed, they were listing the 300W Enhance power supply as being recommended for all speeds of Durons and AthlonXPs, and the Enhance is pretty affordable.