Power supply wattage for a PC

Here is a question for the pros out there, How do you calculate the actual wattage needed for a computer system? I know alot of people say 400watts is good for a new system but what if you plan just to run a hard drive and a dvd drive in addition to the normal motherboard items (sound, video, lan, etc....)?

here is the best PSU calculator that I always recommend:

it is supported by power gurus over at Hardocp. remember total amerage is more important then wattage, so don't fell into the advertising trap of some PSU manufacturers.