Techwarelabs Community - Extreme Power Supply Calculator

This is really neat, can't believe I didn't know about it until today.

"The recommended total PSU Wattage gives you a general idea on what to look for BUT it is NOT a crucial factor in the PSU selection! Total Amperage Available on the +12V Rail(s) is the most important, followed by the +5V amperage and then the +3.3V amperage. Example: PSU you want to buy has 400W total wattage output but only has 16A on +12V rail. But your configuration might require 20A on +12V rail."

Check it out here.