I wouldn't buy a used Duron when you can get a new Athlon at mwave.com for $40:
Mwave Athlon CPUs
They even have an Athlon MP for around $30, MP is for multi processor, it would take some investigation to see if it works on a single CPU board:
Athlon MPs
They do have one
Duron, but it's only $7 less than the Athlon, and IMHO it's not worth the performance hit. Even if the Duron were twice the clcok speed (in this case it's slower), the Athlon will out perform it in most cases.
Hope you can get that baby up and running!