Andrew,
Your project should work in iMovie HD 6 assuming that your not using any third party plugins or codecs that aren't compatible with the new version.
So, just in case, make sure you keep a backup of your original though. In your case I guess that means just keep the old PowerBook near by.
Ordinarily I don't recommend upgrading in the middle of a project unless you have a compelling reason to do so. Which it sounds like you do.
Matt