I already replied to your question over at CadTutor, but I'll post my reply here as well:

The type of work you do will determine what 3D modeling software will best suit your needs. There are dozens of modeling programs and rendering engines. You will have to be more specific with your question.

Engineers might use Solidworks or Inventor.
Architects might use Revit or Autocad Architecture.
Designers might use any number of different programs depending on what they're designing.

The type of rendering software is determined by the modeling program you're using. Not all rendering software can be used with all modeling programs.

There is no one program that stands above the rest, but again, you will have to tell us what specific industry you're interested in. That will be the determining factor as far as which program to use.

You said you want a rendering plug in? Plug in for what? What program?

Architects and interior designers might use Autocad or Autocad Architecture or Revit or they might use a combination of Autocad & 3D Studio Max or they might use Autocad & Sketchup & Vray or they might use something else? If you ask specific questions you will get specific answers. Right now you're just asking very vague and open ended questions that can't really be answered specifically.

I'm a Landscape Architect and I use Autocad and 3D Studio Max for architectural visualization projects. I use Mental Ray for rendering and Photoshop for post processing work.

Your profile says you're using Autocad 2012, is that correct? If so, then why don't you just use the built in Mental Ray rendering engine that comes standard with Autocad? It can produce very nice renders.

If you're looking for a plug-in, Accurender is the only one that I know of for Autocad: http://www.accurender.com/