The fans are controlled by Catalyst Overdrive. Fan speed is 100% when mining with CGminer.
I also read that Diablominer is best for 79xx but it seems it doesn't work for me. With the two 7970s I get only 60 mh/s.
The case is open, airflow is fine, room temperature is low.
Isn't it weird that one of the cards stays cool, and the other gets hot?