Many empty cases I see for sale, and labelled athlon ready, contain a 300 watt psu. Personally I would upgrade to 450 watt, possibly with variable fan speeds.Maplin have special offers from time to time. Athlons run hotter than pentiums, so cooling is important.
Suggest you start with upgrading the psu, and monitor the temperature under intensive use.AMD publish guidelines on cpu temperatures, and if you find you are running near the upper limit, then look at additional cooling.Movement of air inside the case is important, which is why round connecting cables are gaining favour.
My own athlon 2200 has 450 watt power supply, a cheap fan on the back (taking up an expansion slot position) and a fan assisted hard drive heat sink.Temperatures under intensive use rarely go above 60 degrees.