I'd recommend 1x6s for the sides but don't skimp on the plywood bottom. Either way, expect it to rot fast in our PNW weather, which could be a problem if it dumps your garden on top of your garage. Of course, creative use of rubberizes paint on the inside and drainage holes could help tremendously. Not particularly "green" I know, but it was my first thought.

Good luck and let us know how it goes neighbor!

Here is a link that might be useful: Sinfonian's garden adventure

Assuming good drainage, the soil if watered properly will weigh a good bit. I would take a known volume of soil. water it as if it were in your garden and weigh it. Then you can determine what your bed would weigh without any structure. Your next problem will be determining what weight your garage roof was designed to take.

John

Here is a link that might be useful: Johns Journal

There was a thread on the SC&M forum not too long ago with much debate about the weight of soil for a similar project. There could be some info that is useful to you there.

Here is a link that might be useful: SCM soil weight thread