He's kidding about the 280s.
Anyway, SLI is supposed to only work on NVIDIA chipsets. A while back, though, they supported it on a few Intel workstation boards with Geforce 7-series. So, there are old drivers out there that will make the 7300s work on a Mac Pro in SLI, but it's only those old drivers with those old, wimpy cards.
For ATI to compete, they allow Crossfire on an chipset, including the one found in the Mac Pro.
Of course, both of these work only under Windows on a Mac Pro.
Thanks for the info. So, I can install two ATI HD 4870 and crossfire them. If I choose the Nvidia 260GTX or 280GTX, I can only use one card. Am I right?
The Skulltrail motherboard achieves SLI by including 2 NVIDIA MCPs. It'd be kind of silly for Apple to have included these just for Windows.
Who said just for windows? People want OS X support for SLI and crossfire.
Be thankful that CrossFire works in Windows on the Mac Pro at least...
Who said just for windows? People want OS X support for SLI and crossfire.