I typically design using cylinders at PD (as Greg described). I end up changing my gearing approximately 100,000 times during each design process so these approximations make my life way easier.
Once I've finally finished the design and I want to drop in "real life" models I go to
www.bostongear.com and use their BostSpec2 service to download the models for the gears I'm using. (We don't typically make our own gears, we just buy Boston Gears and modify them.)
Check out
www.bostongear.com they have some great stuff.
Then again... lately I just use AndyMark products as my base. They are great and easy to mod for our uses.
Hope this helps!
Regards,
John