While it is possible to run 10.5.6 on that little RAM (So people who say its impossible I can say outright are wrong - I have it on 128MB RAM on my 700Mhz eMac, and it runs), it will be diabolical.
To get it to run on my eMac (Which meets absolutely none of Leopards Minimum Requirements), I installed it using FireWire Target Disk Mode on the eMac, and a PowerBook G4 to install it onto its harddrive - a similar process would work on your PowerBook. However, this comes with a grave warning: on the eMac, opening iTunes takes an ice age. Even on a 1Ghz G4 with 256MB it will run diabolically slowly, but it is possible.