Yes, you can do just what Eidorian says. However, if you have another drive either internal or external I suggest cloning your entire over for protection. You'll also want to have a spare drive or partition for "Time Machine" as backing up to the same disk is not safe.
I plan to install Leopard on a blank partition of an external firewire drive. Then, test it throughly. When I'm satisfied that it is set up and running well and that all my peripherals are happy with it, I'll wipe the internal (I'll still have a clone of that on another disk) and clone over my 10.5 System.
-mj