As rdklinc suggess you should not install Leopard on a 400/500Mhz machine. The machine performance will be sluggish but if you want to try just take off the hard drive from the Ti, put in in a 2.5" firewire enclosure and use a Leopard PPC compatible machine to install the system (using a retail disk) or via a cloning software like SuperDuper. Then reinstall the HD in the Titanium. Another method I think would be to connect your Ti to another Mac in firewire target disk mode and install/clone Leopard to your internal Ti HD.
IMHO Tiger would be more suitable for your PowerBook model.