First, you might want to confirm your GPU cooler is getting adequate and reasonably cool airflow. Some cases are rather poorly designed in this respect.
If your card is still getting too hot, the best thing would be to increase the fan speed. That way you won't lose out on any MHashes.
Or if you don't mind your card running slower, and your card allows it, you could lower the GPU voltage. This will probably require you also reduce the core clock speed, hence the running slower part. but both the reduced core voltage and reduced clock will lower power consumption and should also reduce temps but then the fan might slow down some negating some of the temperature drop.
Finally, one should note that gaming grade GPUs like your 5850 are designed to run for extended periods at relatively high temperatures. As long as all 4 temperature readings in GPU-Z are staying in or bellow the low 80s Celsius, your card will likely outlive it's usefulness (no guarantees though