If you find that you do want 3D, there is a free version of Google Sketchup 2013 available. It also has the facility for 3D modelling. I've only dabbled with it and can't be bothered to spend the time getting to grips with it. But if you're au fait with Autocad I would expect your learning curve to be somewhat shallower than mine.
I also have Draftsight, FreeCAD, TotalCAD, etc. All have their pros and cons, depending on your requirements.
John