I have a
E3110@4 ghz/4870/2 hdd/1 dvd/2 hsf/3 case fan/2x2gb ram and it rarely draws more than 300 watts. I think you could get away with a 450. Calculate here.
http://http://web.aanet.com.au/SnooP/psucalc.php
Also here is a quote from wikipedia:
"Computer power supplies are generally about 70–75% efficient.
[2] That means in order for a 75% efficient power supply to produce 75 W of DC output it would require 100 W of AC input and dissipate the remaining 25 W in heat. Higher-quality power supplies can be over 80% efficient; higher
energy efficiency waste less energy in heat, and requires less power to cool as a result. As of 2007, 93%-efficient power supplies are available.
[3]
It's important to match the capacity of a power supply to the power needs of the computer. The energy efficiency of power supplies drops significantly at low loads. Efficiency generally peaks at about 50-75% load. The curve varies from model to model (for examples of how this curve looks see the test reports of energy efficient models found on the
80 PLUS website). One rule of thumb is that a power supply that's over twice the required size will be significantly less efficient, and waste a lot of electricity."