The 3860 is not an upgrade at all, you won't notice any performance increase over the 8600M GT. The benefit would be that it isn't doomed to explode at some point, but as far as performance goes, it'd be probably the most embarrassing GPU update in Apple's history, and that's saying something.
I wont even mind if it performs exactly the same as the current 8600m gt as long as its an ATI card.
8600m gt card has very nice performance but I would think that apple would use the newest ATI cards at this point.