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.
The HD6950 is an efficient card, especially when factoring in the gaming performance. It consumes around 140 watts under gaming load and only 19 watts when idle, meaning it won't add significantly to your electricity bill.