If you have two 3870s connected with a Crossfire bridge (or 2), I think Mac OS X will still function, just not with the advantage of Crossfire obviously. If you notice, you have to enable Crossfire in Windows, otherwise they just run as two seperate graphics cards. In Mac OS X, whichever one the monitor is plugged into will be used. They're still both in use, just as seperate cards for extra monitors if you like.
A 9600 GT would be silly. It's just a less powerful 8800 GT.
Whoever gets a retail 3870 Mac and PC edition first, could you list off the package contents? I want to know if it has the power cables and Crossfire bridges.