Im a student of architecture and this is what i recommend:
Google Sketchup - For easy 3D modeling with less realism (free version is available!) Can show dimensions
Vectorworks - For accurate 2D plans and 3D modeling, but more complex, a degree of realism. Can also show dimensions
3D Studio Max - For the most realistic 3D modeling, more like Final Cut compaired to iMovie (Sketchup). Can do anything, they made shrek on it but it can be very complicated.
I use vectorworks, but plan on learning 3D Studio Max. I have attached a render I made, I know its not brilliant but it helped me pass the year!