If you don't want to buy a new PSU right now, first try it with 300 Watt, if the system runs fine (stable), you can either choose to get a new PSU just in case you're getting some more peripherals, or wait till you start adding more cards/peripherals and notice a decrease in the stability of the system (especially lockups and sudden reboots).
A 400 Watt PSU should be more than enough for any 1.4 GHz Athlon system.