If I recall correctly, fan speeds could be manipulated by means of a slider in ATi's Catalyst Control Center. It's fairly crude though and only allows for a fixed RPM rather than the ability to apply xx% fan duty at x celcius and so on. At full pelt, the fan is no doubt quite loud but in a case with sufficient airflow, this shouldn't be a necessity.
I haven't got a 5870 on me nor even a suitable testbed (back to uni with just my ITX PC right now) so I wouldn't be able to confirm that. I am very sure that I saw a fan speed slider in the Overdrive tab.