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.
With the current limiter disabled the card demands 322 watts running Furmark ‘burn in’ test. When gaming it is a more modest 239 watts under load.