The powerball is good for getting into nooks in the wheels, but might not be up to the task...depending on how bad they are. I've heard good things about this:
http://www.meguiarsdirect.com/product_detail.do?q=MEG+G13508
It all depends on how much time you have to polish them, it can take a while depending on how dull they are and how convoluted the design of the wheel is, etc. I've seen some people wet sand them with fine grit lightly before polishing or use fine steel wool with compound...