In NP3, whenever I set the number of cores above 8, NP3 dies on Fast Render and Background Render. That's fine, I guess. 8 cores cranking is still crazy fast. But not as fast as disabling NP3 and letting AE render using multiprocessing (which opens up to 24 instances, if I'm understanding the way AE handles hyper-threading correctly). The Spec options seem to work fine when assigning a higher number of cores.
Now I readily admit that I don't have a good handle on how AE CS5 deals with these processors, especially when you throw in NP3. Maybe I should take heart, as it seems that Adobe hasn't really solidified what constitutes best settings for the multiprocessing settings in CS5 either.
I feel like I should understand all of this stuff and that there's something really obvious that I'm missing ... but that's what the Cow's here for, right?