“Satisfactory” is a very subjective term. All depends on things like frame size, what filters/behaviors/animations you apply to your objects, if you start using particles or the replicator, etc.
I do a fair amount of demos of Motion on my Powerbook 17″ and after working on a dual G5 with an x800 card it is painful (before that it was adequate).
It should be sufficient for simple composites and animations, but you will find yourself wanting to explore more aspects of the program and then you will feel pain.
Jim Kanter,
Digital Film Institute
http://www.dfilminst.com