A dual processor G5 is definitely going to give more juice than a
single processor machine.
The real problem with Reason 4.0 and Motorola processors is with
running Thor.
Thor uses very complex calculations that really require an intel
processor.
I have a G4 Powerbook 1.5GHz with maxed out RAM and have major issues
with running Reason 4.0 in standalone with a few instances of Thor....
I don't know what your projects include. From your description, I
would venture to say that you are trying to run a number of Thor
instances.
If you can get away without using Thor (I hate saying this), you will
probably have better performance.
I will be showing some ways of getting around a maxed out processor
tomorrow using the sample playback capabilities.....
I don't think that you need to go back to Reason 3.0, but be mindful
of what in 4.0 will cause problems.
Definitely try the advice of Trailmix as he is using PT8 on a Motorola
PPC and I am not. I can only speculate as to what the possible problem
may be......
Hope this helps!
cp