I recommend you to use google sketchup. It is easier, with possibiliy to export to autocad. There is also Blender which is very good as well, more difficult at first sight, but difficult programs has their advantages as well. Other good programs exist as well, choose one you are comfortable with: 3d Studio Max Papervision Maya etc are one of these. 3D studio max is very popular, I guess it is because it has success in 3d design by so many people. But it doesn`t mean it can fit all your needs. So this mean, try and then choose a program afterwards, Autocad has lots of improvements but still designers use 3ds max for architecture. Maybe that`s why autodesk bought the 3d Studio Max link for 3d programs http://www.software4free.org/3d.html
Do not waste your time on designing all part of building, ask yourself which part of the building are you willing to render. just design the viewable parts.
