The temperatures of Diamond's Radeon HD 7870 Double Black Diamond card were very low. Based on AMD's reference design, the cooler seems more than up to the task of keeping temperatures low.
Thanks to the new 28nm manufacturing process, the Radeon HD 7870's power usage was significantly lower than the last generation Radeon HD 6000 series GPUs. The only card using less power was the 7870's little brother, the Radeon HD 7850. This should be no surprise though since the 7850 only requires the use of one supplemental six pin PCIe power connector (versus the 7870 which requires two).
Ending this section with noise, I don't yet have a standardized practice in place studying the noise output of hardware. But after using the Radeon HD 7870 while gaming, I do have some subjective observations. When idle, the cooler is essentially silent. With the load of an intensive game, things change on the noise front and the fan speeds up.
The cooler on Diamond's reference design based Radeon HD 7870 is capable of keeping temperatures low, but this also results in the blower needing to ramp up to higher speeds. When playing games, this results in a moderate hum. It's quite at an annoying level of noise, but it's definitely noticeable. Looking at the cool temperatures we were able to achieve, we feel that Diamond should adjust the blower's cooling profile to lean more towards quietness at the expense of higher temperatures.