"I'm not sure we're supposed to know this, but as long as you have the proper power adapter (you'll have to buy it online), and Lion, the Mac Pros will work with any off-the-shelf ATI Radeon HD 6000 series card"
Really??..what about the drivers? Would the card be 100% functional (all settings available)?
Any chance that could work on Snow?
The video I posted is fairly helpful:
It states that the card doesn't display a startup screen, so it's not really fully functional, but he could always keep one of his GT 120s in the machine as a spare for that purpose.
I believe Lion contains the driver for the 6870 because Apple uses the mobile version of it as an option in one of the new iMacs. I wouldn't expect it to work with Snow Leopard I'm not sure what settings you're describing. I haven't done extensive testing, so I'm not sure I can vouch for how well the card actually works vs Apple's 5870, but this is MUCH cheaper.
PCs are finally adopting EFI, so it's possible we could see much greater card compatibility in the near future.