Exeter Acres is giving you some really good advice here. Think about it: You're asking about an expensive gismo that you dion't really need and trying to use some cheap servos....and you said for a 3D system. If money is tough there then I'd skip the fancy power expander and spend it on some better servos. Some Hitec 5645's would be great.
http://www.servocity.com/html/hs-564...al_torque.html