XFX HD 7970 Black Edition Double Dissipation 3 GB Review @ OCC
 

Keep in mind, the base clock speeds of 1000 MHz on the core and 1425 MHz on the memory are already sizable increases over the default clock speeds. However, XFX has added their Double Dissipation cooling to increase the overclocking potential of their Black Edition HD 7970. As a result, the card responded well to voltage tweaking and a further boost in the clock speeds to 1177 MHz on the Tahiti core and 1652 MHz on the GDDR5 memory. Clock speeds above 1.2 GHz on the core were possible in some games, but not the entire test suite – Dirt 3 just would not show love for a core speed that high.

http://www.overclockersclub.com/revi...black_edition/