if you dont have enough ram honestly dont bother to upgrade your powerbook g4 867 mhz titanium to leopard , leopard needs at least 512 and that is the absolute minimum ram to be usable ..sort of !!! your powerbook takes a max of 1 gb so its time to upgrade even if you stick with tiger because tiger runs with 256mb ram but its already for tiger a form of torture , you will see a huge improvement in speed with more ram
for the OP the G5 should run leopard , but as mentioned now i would not run it with less then 2 gb ram as minimum even on a G5 as it felt slow on my iMac G5 with 1.5 gb ram compared to tiger which did really fly on the G5 iMac