The Mac Pro does not have a SLI certified motherboard, and Apple's Nvidia drivers also do not support SLI. When in bootcamp, you still cannot enable SLI.
I tried to experiment with hacked drivers in bootcamp to enable SLI, but nothing that i found or tried turned out to be real.
You'll have better luck going with Crossfire/Eyefinity in bootcamp with some ATI/AMD cards. Although you will have to do some monkey-ing around with a secondary PSU for the second GFX card if you are going to use a 5870. Already a couple people messing around with it on the MacRumors forums.
softTH is an option that will work, although it does take some toying around with to get the configurations to work out, another option would be to try the good 'ole Matrox TripleHead2Go, but you will likely have to drop to a lower resolution (1680x1050) on the monitors, but your aspect ratio will be a little off.
-Ray
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