I have owned a x1950gt in the past and that was quiet unless i had it at 100% fan
i now have my 4870 settings at 32% fan constant (2100rpm) and its about the same from what i remember, but at stock settings the the fans on the 4850/4870 run at low RPM's for 95% of the time then sometimes when the gpu is under load the fan jumps up to 50% making it loud for about 10 seconds
hope that answers your question