The first app to have if you don't own it already is Photoshop. It's the must have application if you are into 2d or 3d art, Deep Paint3d is great but expensive, when you work in it you are always sending the material back and forth to photoshop via the DeepPaint to Photoshop plug-in, some things are just easier to paint flat. These two applications work very well together.

Dominique Digital Cats Media