To test power consumption today we are using a Keithley Integra unit and we measure power consumption from the VGA card inputs, not the system wide drain. We measure results while gaming in Crysis Warhead and record the results.
This article is also worth a read if you own a GTX580 – GPUz can be used to disable power throttling. Our results are taken with the limiter system in place although if you disable this, the card can consume over 300 watts of power.
The Asus GTX580 Direct CU II is taking 2 watts less than the reference GTX580 when idle. When gaming, this rises to 225 watts which is a pretty good result considering the power on tap.