I used a similar method to get Leopard onto my wife's 800 MHz iBook.

1. Use SuperDuper! to make a clone the iBook hard drive on an external Firewire HD (that way if anything goes wrong I can take the iBook back to Tiger without loosing a thing)
2. Fresh install of Leopard on my MacBook (erase and install), with a recent Time Machine backup from which I could restore all my files.
3. Start up the iBook from the external HD and the MacBook in Firewire target disk mode.
4. Use SuperDuper! to clone the MacBook internal HD to the iBook internal HD
5. Use the Time Machine backup to restore the MacBook to its original state and the Tiger clone on the Fire Wire HD and Migration Assistant to get all of my wife's data onto the internal HD of her iBook, now running Leopard.

We have three external HDs, one for my MacBook and two for her iBook, one for Time Machine and her iTunes media (her iBook only has a 30GB internal) and the other still has the Tiger clone so that if she wants to she can revert back to Tiger.

All of this took about a full day, but as of right now everything is running great. And if something doesn't work well we can always go back to the way it was.