As far as I know, windows64 can handle at most 2 independent cpu's, and the kernel task scheduler for windows still sucks bigtime, which has been mentioned in the reviews of the dual core prototypes, so does the thread limit windows currently has.But, OS X,linux, and the BSD's dont have this issue, theyve been multiprocesssor for quite some time.
Gonna be real interesting to see what apple's reaction to this will be cause a dual core running a little faster, say maybe next generation of these things, will make the dual G5 look like a punk toy LOL, and be competitive or even cheaper bang for the buck wise by quite a bit.
IBM has taken the PPC architecture almost as far as it can go with the G5, wouldnt shock me if apple made the switch in a year or two, that would certainly change things alot.I havent heard any rumors yet of what IBM's response to this is just yet, I bet they are watching it closely.Intel got burned and so did windows when AMD went ahead with the 64 series even without their blessings.
Just a guess at this point, but multi GPU vid cards are near their limits too, imagine them working in parallel with multicore CPU's !! I think that is what the future will be, and its not far away.