Look for the biggest you can get. 550 watts should do you fine. here is a selection and prices, pick the show all products tab. p.s. don't worry if it says P4 as these just have extra motherboard connectors.
Fitting a new psu is very simple. the psu is attached with just 4 screws. If you make note of the mother board,ide and floppy connections before you remove it, the change over is very simple.