never used the 8 core but i can comment on the 4 core macpro if that helps..
for most of my not too complex scripts, rendering will not be faster when using all 4 cores, in fact usually it will be slower..
for example a typical render could look like this
-cpus 1 renders 2.9 sec / frame
-cpus 2 renders 2.4 sec / frame
-cpus 3 renders 2.5 sec / frame
-cpus 4 renders 3.1 sec / frame
(-cpus 1 will use all each processor to 25%, -cpus 4 usually taxes all 400% available)
however, if i launch 3 instances at the same time with:
-cpus 1 -t 1-100
-cpus 1 -t 101-200
-cpus 1 -t 201-300
the total render will be nearly 3 times as fast while still being able to do light work in the gui.
so basically, if you work with image sequences and have fast disks, 8 cores sound like a good idea. (but on the other hand, with that money you could nearly buy 2 quad comps). i guess with a good render manager you could get some additional performance out of it.
++ chris