SketchUp is free from Google just Google it and download to your computer. You can do just about anything with it, but it is especially good for conceptualization of architectural design. The current version (November, 2010) is 8.0. These instructions are based on this version. There is also a professional version, SketchUp Pro 8.0, that they sell online for $495.00. Students can purchase a one year license of SketchUp Pro for $49.00. But I would recommend downloading the free version from Google.
The major difference between free and Pro version is the ability of SketchUp Pro to import from and export into AutoCAD. If you really want to import AutoCAD drawings, you could download the SketchUp Pro version for an 8 hour trial, import the AutoCAD drawing, and just not follow through with your purchase.