Now for some power and noise testing. Notice that the cards marked with asterisks in the results below have custom cooling solutions that may perform differently than the GPU maker's reference solution.
The 6950 and 6970 draw a few watts less at idle than the GTX 570, fitting for a smaller chip. When running Left 4 Dead 2, however, the 6970 actually pulls a little more juice than the GTX 570. (We expect one might find different results with a different sort of graphics workload, but we think L4D2 is a good, representative game with relatively high power draw.) The 6950 is kind of in a class of its own, but its power draw is relatively low under load, only slightly more than the GeForce GTX 460 1GB's.
Noise levels and GPU temperatures
Nearly all of the single-GPU solutions are pretty quiet at idle, and most are perilously close to the noise floor for the rest of our system's components. Still, the 6950 and 6970 prove to be exceptional citizens, among the quietest solutions we tested. Dropping in a second 6970 does raise the noise levels at idle a bit, likely due to the obstruction of airflow into the primary card's blower.
Under load, Nvidia's stock coolers simply outperform AMD's. The GTX 570's GPU temperature exactly matches the 6970's, the two cards' power draw is only 5W apart, yet the GTX 570 is 2.5 dB quieter.