Leopard can be installed on G4 Titaniums, but only ones that have at least 512MB of RAM, and a minimum processor speed of 867MHZ. There is software that can trick the OS into installing onto a computer that does not meet the processor speed requirement, but believe me, with a 400 or 500MHZ G4, Leopard would be PAINFULLY slow.
Install 10.4.11 Tiger instead. Compared to Leopard, it's a very lean and quick OS, and it is still supported by a lot of software out there.