Manufacturers usually supply the psu adequate to the requirements of the computer,usually 350 watts and above. Having an higher wattage psu is not a bad idea, if you think that you are going to make additional changes to the existing specifications.If you are thinking of changing the psu, make sure that the replacement will fit the existing set-up, before making a purchase.
If you are still on speaking terms with the computer manufacturer or retailer, then you could perhaps ask them for the specification, or you could post the computer model etc on here.
If you do not know what is in the case, then Belarc click here SIW click here or Everest may provide the details. The first two are free to download and use, Everest is now old hat in the original version or payment required in later versions.