Unfortunatly, there is no 'one way' to determine the power supply you need. [ That I know of anyway. ] The only real way to do this is to go component by component and their power rating, but this is unrealistic.
350watts, used to be a 'standard' but with users adding more and more components, power needs have went up.
Your best bet, (especially if you plan to upgrade anymore) is to get a good brand 400/450 watt ps. It is always better to have more, than not enough. A ps of that size, should have no troubles running everything in a conventional box.