Weeeeellllll . . . I had until recently an upgraded G4 Pismo--G4 550 MHz Power PC--and it could run all the way to 10.4.11--never tried to load 10.5--I think that is for Intel only.
So unless someone knows otherwise, I think you can run up to 10.4.11. This means you need to get either installation disks or upgrade disks. At this stage since 10.3 and 10.4 are "old" and everyone is getting 10.5 and getting excited about 10.6, you might as well get a full commercial installation disk for 10.4.
If you get "update" disk you may need the intervening disks. You can get a full installation disk for 10.4 very cheaply on eBay, on some Mac sites--I even know a dealer who sells them. You can PM me for his contact information.
--J.D.