Leopard isn't really done yet, and is changing and under NDA, so we can't really talk about it.
Your new Mac has 10.4.10, but a different build. It should be able to start up a copy of its own OS, yes.
I don't quite understand your AirPort Express question, but see "Backing up over a network" in the User's Guide, as well as the "Airport Disks" post at
http://www.shirt-pocket.com/blog.