The .Net training provided by Autodesk should get you started, it's 6 or 7 different labs with VB or C# examples.
http://images.autodesk.com/adsk/files/autocad_2007_.net_training.zip
Objectarx is direct access to Autocad Command, Database, etc.. I believe you have to use C++ or C# to do much with it.
The .Net API are manager wrapper classes for ObjectArx. Most everything you can do with ObjectArx is available in .Net and if it isn't you can mix and match .Net, ObjectArx, and COM.