|64-bit EFI ROM for Radeon 6870:|
(clip from hackintosh site Netkas posting on (netkas.org/?p=919) 64-bit EFI ROM for Radeon 6870)
"We've done it again, 64-bit efi rom for 6870 for MacPro3,1 and later. (2008 and later EFI64 models)
It gives boot screen!
Head over to this thread for more info - forum.netkas.org/index.php/topic,1085.0.html
In short, what works: any connection on upper ports (4 ports in line), any allowed combination of them (max 2 legacy (dvi/hdmi, 1 vga)) connections - dl-dvi on dvi port, sl-dvi on hdmi/dvi/mdp (with passive adapters), vga on dvi port with adapter, hdmi on hdmi port. MDP ports work fine with active adapters or native DP displays. Lower DVI doesnt work at all.
Maximum - 4 screens. For example, if you use active adapters, it can be 3 dl-dvi displays and one sl-dvi.
Bootscreen appears if only one dvi display connected to upper dvi connector (single or dual link).
Steam/dvd player work fine."
Back in March there were notes on 6870s working w/OS X 10.6.6 Drivers from 2011 MBP. (As I mentioned then, IIRC the 6870 is a lower-power (and slightly lower performance) version of the 5870.)
BTW - There's already some of these flashed 6870's for sale on eBay.