We measured total system power consumption at the wall socket using an Extech power analyzer model 380803. The monitor was plugged into a separate outlet, so its power draw was not part of our measurement. The cards were plugged into a motherboard on an open test bench.
The idle measurements were taken at the Windows Vista desktop with the Aero theme enabled. The cards were tested under load running BioShock in DirectX 10 at 2560x1600 resolution, using the same settings we did for performance testing.
Check that out! Although the Radeon HD 3870 performs almost exactly like the 2900 XT, it does so while drawing vastly less power. Our HD 3870-equipped test system draws 98W less than our otherwise-identical 2900 XT-based rig does while running BioShock. That's a massive honking reduction in power consumption versus AMD's previous-gen chip. Not only that, but the HD 3870 system pulls 21W less at idle and 39W less under load than the GeForce 8800 GT system.
We measured noise levels on our test systems, sitting on an open test bench, using an Extech model 407727 digital sound level meter. The meter was mounted on a tripod approximately 14" from the test system at a height even with the top of the video card. We used the OSHA-standard weighting and speed for these measurements.
You can think of these noise level measurements much like our system power consumption tests, because the entire systems' noise levels were measured, including the stock Intel cooler we used to cool the CPU. Of course, noise levels will vary greatly in the real world along with the acoustic properties of the PC enclosure used, whether the enclosure provides adequate cooling to avoid a card's highest fan speeds, placement of the enclosure in the room, and a whole range of other variables. These results should give a reasonably good picture of comparative fan noise, though.
These measurements probably aren't precise enough that it's worth worrying over minor differences like tenths of a decibel. The things to take away from this are the big differences, because those are the ones you'll notice. Such as: the Radeon HD 2900 XT is rather loud while running games, as is the GeForce 7900 GS. The Radeon HD 3850 and 3870, meanwhile, are both as quiet as they come.