Jmac,
You can stay with Cheetah3D for a while. it is a pretty good modeler. I prefer Hexagon but it's more a matter of preference in user interface. You will find that most all of the tools in Cheetah are available in most any modeler, and work pretty much the same. So any knowledge you get out of it will be transferable to other applications.
I agree with bwtr and Grendel. Zbrush is REALLY cool, but it's probably not the best place to start, if you plan on using 3d to reconstruct scenes. Plus us Mac people are still waiting for ZBrush 3
Carrara, is a good rendering option, and can be used for lots of things, including animation. It has a pretty low learning curve. and as mentioned, the free Bryce and DAZ are a very good place to start. And you can find several good forums for getting your questions answered
as for Blender....Grendel is right, if you have NO experience in 3D you might be able to figure it out.