Hexus published a little guide that shows you how you can adjust the fan speed of the Radeon HD 4850 and 4870 graphics cards by editing an XML file:
AMD's Radeon HD 4000 series graphics cards are impressive on most fronts, but, they're a touch hotter than most users would like - particularly the single-slot Radeon HD 4850.
The heat-factor isn't helped by AMD's lacklustre automatic fan control. Up until now, users have been unable to manually control the speed of their fans.
That, however, is no longer the case as a forum user going by the name of bretware over at Guru3D has provided a useful workaround that'll let users manually control their card's fan speed, and consequently lower temperatures.
Running the fan at 65-70% reduces the temperature by as much as 30°C but it increases the noise of course.