I don't think the motherboard/chipset supports SLI therefore whether Windoze or Mac it won't work.
i was a bit anxious upgrading mine given some of the feedback in here and the support for the new ATI card coming soon, but I am very pleased I did. See no negative effects in OSX, the Pro Apps I have seem as snappier as ever, and the games in Mac, and especially in bootcamp, are much quicker.
And most importantly it is nearly silent - 3 hours of HL2 in bootcamp and the dodgy music was still scaring me at times whereas with the X1900 I struggled to hear what was going on over the sound of the fan blowing like a hairdryer!
If I was being very critical I think that OSX is slightly softer in image than the X1900 but I suspect that is more because I was trying to find something negative rather than it jumped out at me as something that was a concern.