Here is a link to a power supply calculator:http://www.antec.outervision.com/
Just fill the blanks as much as possible to come up with an estimate for the total system. Please bear in mind that the result will be overestimated because there are a lot of cheap, low quality psu's of questionable reliability that people insist on buying. If the cpu is the brain of a pc, then the psu is the heart. No point in risking premature death due to heart failure.