i think yours is a 300watt...
Normally people use 400-500W power supply for general purpose. if you have a high range graphics card or sound card you might need a 600W.
If you go to a shop, rule of thumb is the heavier the weight of the thing, the better it is.
If you go online i'd recommend http://www.ebuyer.com, look for power supplies and look at which ones get 5 star ratings + reviews.
If you leave your old powersupply plugged in you should be able to just match where the wires go. 1 to motherboard, 1 to cd drive 1 to each hard disk etc etc.