I've also put across the (nearly) empty Project Crystal directory to see if it's painless to run more than one project in the same repository, and it seems to work. The starting revision number is already up to about 30 so I'm assuming it's based on the entire repository rather than the project.
I suspect it'll take me a while to figure out how to properly use the differences between the "trunk", "branches" and "tags" in the directory structure that the documentation suggested - I'm not sure what's standard practice for when to use a branch or what to name the tags - and I'm still unsure of the best way to backup the repository, but at least now I don't have to be so paranoid about breaking something with a sweeping architecture change.
To back up I do svnadmin dump. This blog entry has some more info.