I recently was able to install the FireStream 9270 card into my Mac Pro and run it properly under Windows Vista 64-bit. I had originally tried running the card in tandem with an NVIDIA card only to realize that Vista can't handle both. I had to totally remove the NVIDIA card and remove all traces of the drivers and restart the computer --- but now it works!
In addition, I had earlier removed all the BootCamp drivers off of Windows, so I'm not sure if that influenced my success or not.
BTW, the Mac Pro was the old 2006 version, so if you are using a Mac Pro you must make sure to install the card in the first PCI slot (the only x16 one available).
In addition, you will need to 2x3 6-pin power connectors to get extra juice from special plugs in the motherboard. There are only two companies that sell these cords (they didn't come with the card). I got mine here:
You will need two of these (kind of pricey but necessary) plugged in from the Mac motherboard to the card, or else the card will not power up properly and the fan will run full blast (indicating the card is underpowered).
Hope this is useful!
Now, if anyone can tell me how to mix/match ATI/NVIDIA cards under one roof (cough cough, Vista) then I'll be happy