I use the Zalman VF1000
Newegg.ca - ZALMAN VF1000 2 Ball VGA Cooler
It is not supported for the 5850, but the cooler fits on the card without any modifications, and keeps the card very cool. Even at full fan speed, it is very quiet.
This thread shows you how to install the cooler for best effectiveness:
Better cooler for ATI 5850 - Zalman VF1000 - Futuremark / YouGamers Forums
Good luck.