Klyph is right. Google SketchUp is the easiest way. I believe they have a new one out, SketchUp Pro 7. It is very easy to use, short learning curve and many models available in the on-line 3D model library (kegs, march pumps, CFC,etc).
I finish my design and starting to acquired the material for the build.
Let me know if you want a copy (for reference)!
Good luck and cheers!