For the money CAT is not really worth it,its main use is to create a pipeline for getting a lot of motion capture into Max, although it is easy to create animated motion using this software its still relatively easy to do this in Max without it.The standard biped in Max can be manipulated in many different ways to accomodate even quadrapedal characters and with the ability to attach extra bones for limbs,wings etc.Use the motion mixer or motion flow mode.
Cat doesn't add much to the equation but as it already has a range of rigs for creatures of all sorts it can be useful,I would suggest getting in about the controllers,wiring and layers that Max already provides.
Bond1,I was thinking myself of purchasing Mudbox,but its just been released and I haven't seen a demo download available for this yet,this has put me off a little as I'm not going to buy unless I can try it first.
Save up for a Harley Locrian,you'll get better motion