You will need to know a little about the insides of your PC to calculate consumption.
There are various calculators on the web, here's one: click here
The idea is to put a tick in the relevant boxes and it will automatically calculate the wattage needed.
Don't think because your 500w self destroyed because it wasn't enough, or that your 400w won't be able to handle it - it all depends on how old it was, make etc and just how much energy you are consuming at maximum. If your PC is, and I say this politely, average, then 400w should be sufficient.