Power demands

From TheBestCaseScenario

The power supply is the heart of your computer. Don't be afraid to pay a little more to buy a high quality one such as an Antec, OCZ, or Corsair. I've also had luck with some other brands, but be sure that you read reviews on the power supply you are trying to purchase.

Newegg.com has a very good wattage calculator on their website : http://educations.newegg.com/tool/psucalc/index.html

Just keep in mind that this calculator only calculates the power of the components listed. Fan controllers, lights, fans, and other devices need to be factored in. I usually go round up to the nearest 50watt and add 50-100 watts above that to leave room for upgradability.