Followings are the overclocking profiles of my ASUSTeK MATRIX HD5870 P/2DIS/2G
I generally flighsim under 970MHz setting but sometimes further OC to 1000MHz as follows:
Fan speed varies from 40%, 50%, 70% to 100% according to the temperatures set.
Both profiles are not aggressive and quite stable under my system. However, I prefer 970MHz more since it keeps the noise of the fan to a lower level most of the time.