Ok - first of all - it will work with a Mac Pro. I had a AMD Radeon 6870 w/ flashed bios in my Mac Pro for several months before I decided to retire the system. The video card works fine EXCEPT you have to know it doesn't give PCIe 2.0 speeds. Also, the flashed version will give the gray boot screen (allowing you to optionally boot to windows or OSX on a dual boot machine) when a monitor is connected to the DVI port. The non-flashed version should work (reports have been found of it working with Lion and Mountain Lion) except you will not get the boot gray screen (although I had read in once place where ML changed that, but I can't confirm this report).
The fan on the card is louder then the 8800gt I had in my Mac Pro. I never tested it with DVD Player (some people had reported issues with the DVD Player app) but never had problems in my other apps (games, FCPX, handbrake, safari, etc.)
The card also worked awesomely in Windows for gaming. It was a little low on performance compared to official benchmarks but I figure that was due
You can see in the attachment my novabench results with the card in it.
There was one other flaw, but it's a flaw with Lion more then the card (it's a flaw that was reported on Apple's boards with regard to the AMD card in the 27" iMac's that also affects this card) - with the card in (at least as of 10.7.3, I think of the problem was on 7.4 as well, I just don't remember if I tested it), on a fresh boot it works flawlessly. If you let the computer go to sleep for an extended time (a minute isn't long enough as there is enough residual charge to keep settings in place, I don't remember what the minimum time was but I remember experiencing it after like 10 minutes or so of sleep) and wake it up you'll find the gpu performance has dropped dramatically. Some people had success with just changing resolutions to get everything to reset, I had to actually reboot my system and eventually just stopped it from ever sleeping. I don't know if this issue was fixed in Mountain Lion. Here's just one thread on the subject at Apple :
https://discussions.apple.com/thread/3203487?start=0&tstart=0
Here's a link to someone who put (I believe non-flashed) version of a 6870 in a Early 2008 Mac Pro:
How to install a 6870 Video Card into a Early 2008 Mac Pro 3,1 running MacOS X Lion or earlier release.
