Rather than go by a wattage rating, which can be deceptive, I'd suggest you go first to AMD's site and dig up the info on how many amps the Duron 700(or higher if you plan to overclock) is going to pull on the 3.3V line, then check out the specs on the power supplies you're considering. Be sure to leave yourself a little margin beyond what you figure you'll need, it's not a good idea to push any component to its max all the time. As far as all the other power supply voltage lines, you'll need to take into consideration all the other hardware in the system before you can make an informed decision on that part.
Paul