I just installed Tiger on a Powermac g4. I used the Migration Assistant to transfer everything from my Powerbook to the Powermac. It transfered all the folder I told it to, all the settings, and all my applications in my applications folder. It was very easy. All you need, as said above, is a firewire cable.
However, these cables can be very hard to find, as you need one with the large end on both ends. I went to 5 stores, until I eventually found a Best Buy that had one. However, it was not located in the computer section, it was actually by the camcorders.