Hi there, not using the same card as you but am using a 4850X2 which is cooled by two fans. At start up/idle it tends to be running at about 47° - 50°C.
When doing something which is heavy on graphics and with the cooling fans going flat out I have peaked at about 85°C. Having looked into this a little bit it seems that these chips do run hot.
If you are worried about the temps, have you considered a third party fan to add some extra cooling? Being a passive card you are dependant on the airflow through your chasis to keep your temps low.
Just another thing to consider, are you running the latest drivers (ATI Catalyst Version 10.2).