ok, so when you say "much lower fan speed", how much lower???
Plugging out the power of your HD5850 doesnt help because my computer starts to beep like mad.
Here is how I solved it: My Gigabyte GA-P55 UD3 gives me the possbility to change the main PCI-E Slot (PEG1, PEG2 or AutoPCI)
In the first slot I have my HD5850 (in PCI-E 16x)
In the second slot I have an old XT1900.
In OSX the XT1900 has to be enabled with EFI String and in apple.bootlist the GraphicEnabler is set to NO. (don't forget to repair the permissions)
This makes the HD5850 to switch in a much lower fan speed.
In W7 you have to disable the XT1900 card in System and reinstall eventually the drivers.
Right now when I choose to change from W7 to OSX I startup the PC in the BIOS and choose PEG2 and second Harddisk (my OSX DISC) and F10 Save and Reboot
When I change from OSX to W7. I change in BIOS to PEG1 an first HD (W7) and reboot.
Actually XT1900 works with QE/CI but not Dualscreen
But hopefully sometime we get suppport for HD5850
I mean, is it still audible?? anoying? not for working??
still, I'm really happy to see this alternative coming to life