Over the weekend, with Joel's and Mike's help, I tilled most of the back garden, removed a whole lot of rocks, laid out the first beds and started spreading the composted horse manure.
So I'm now committed to the following layout, which you can see schematically represented below. Three sets of 3 pairs of beds. Each bed is 30" by 15', with a 12" path between them. Between each pair and each set is a 36" path. The idea is that 30" is small enough to straddle, and 36" is wide enough for a wheelbarrow. Each bed has a path next to it that is wide enough for a wheelbarrow or wagon and will thus be easily accessible for planting and harvesting.