I have loaded 10.4 on machines as old as the Bondi Blue iMac G3 with 196mb RAM. It runs fairly well considering the equipment that it's on. The main reason behind Apple's 10.4 requirements is that indexing the hard drive for spotlight consumes a lot of resources, but a G4 iMac is more than powerful enough. Load it on there and if it feels sluggish, check for a dot inside the spotlight icon on the top-right corner of the screen. If a dot is there, this means that its indexing your hard drive for spotlight search. Give it some time and wait until it finishes indexing your drive. After that is complete, if it still feels sluggish, check out a guide at macosxhints (http://www.macosxhints.com/article.php?story=20050504012104186) that will tell you how to disable spotlight. If you have at least 256mb of RAM, I'd leave it running as it is an extremely useful feature.
10.4 also lets you run newer versions of Safari, QuickTime, Java, and FireFox. This will open up so many more websites for you and less Safari crashes than with 10.3. Panther was great for its time, but for an older PPC, Tiger is most def. the way to go.
Oh and more importantly for you, Tiger will run iTunes 8...