"We’re now recommending much higher settings for the RAM To Leave For Other Applications and Minimum Allocation Per CPU preferences. If you start with these numbers both quite high, you will avoid the problems of swapping memory to the hard disk (which is a real performance killer) and starving the background rendering processes such that they shut down and don’t do anything."
Update: There's discussion of this on the Adobe Forums, more in an Adobe FAQ, and some on the AE-List where Carey Dissmore began a thread on a benchmark article on Ars Technica,
"Bumpy road to multi-core: Ars reviews the 12-core 2010 Mac Pro, Main article: http://bit.ly/blLbiS
Of particular note is page 4, http://bit.ly/9fZkcT where FCP and AE don't show very strongly on multithread utilization. Looking forward to some future date when these apps are super-optimized. I know development takes time, lots of time, and it's not easy to just multi-thread any old operation. But still…
BTW Premiere Pro is pretty wicked-fast-impressive these days though. Shows what the hardware can do with new code."