I'm on a 2006 mac pro running a XFX HD 5870 which I found on pricelover.com for around £110. Flashed the BIOS to make the card work in OS X. Left clock speeds and voltages as stock.

Get really bad speeds in OS X due to Apples crippled OpenCL implementation.. somewhere around 150mhash/sec last time I tried, although I hear its much better running OS X 10.7 (lion).

When booted into Windows 7 I get approx 350mhash/sec when left at stock settings.
If I overclock with MSI afterburner I can achieve 430Mhash/sec when clocking to 969/350.
The card crashes or becomes unstable (although it recovers without a reboot) if I clock GPU over 970.
Running most recent AMD drivers (11.6 I think) and phoenix with the Phatk kernal.

Similar results (430mhash/sec) when booted into Ubunutu, although I have to flash the bios to overclock out of the preset ranges as I don't believe there's an MSI Afterburner type app for linux?

Temps anywhere between 65ºC and 75ºC depending upon ambient temp.
I leave the side of the mac off and the fans appear to need to do less work to keep GPU and mac CPU cool, meaning less noise.