3d modelling software for case design
Generally, you need software that can export STL files.

Solidworks: $4000 + $1300/year

ProEngineer: $5000? + something per year?

Autocad Inventor: $5200? + something per year?

Google Sketchup: Free or $500, but can't export STL directly. I've seen mention of a plugin that can convert Sketchup files to STL, but I haven't used it myself.

(From Peter Gibson) Blender: Free, but alleged to have a steep learning curve.