If you don't already know about SketchUp, the free** tool from Google, temporarily stop reading this post and point your browser to the SketchUp site to download your favorite flavor.
(Once you start using SketchUp, their blog is well worth checking out as well.)
I know a fellow exhibit developer that calls SketchUp "AutoCAD for Dummies." While SketchUp may have a much more straightforward learning curve than other 3D programs, the results are beautiful and professional even for a clod like me. (The drawing above is a simple SketchUp rendering that I created for a museum project.) So download SketchUp and give it a whirl!
I'd be interested in setting up a network for sharing SketchUp models and projects related to exhibits and museums. Anyone else interested?
**Once Google bought the original SketchUp company they started offering a free "basic" version, and charging for the "pro" version (although non-profits, like museums, can get the pro version at a reduced rate.)