I currently have the sapphire 1gb card - but she runs so hot. I what to crossfire with a second card, but with my card running as hot as it does I wouldn't consider getting a second sapphire. That's why I've been looking into the HIS IceQ 4870 - I hear it runs quite cool and quiet. Sounds perfect for crossfire.
However I wont pull the trigger until I know it will work in the mac pro.