Hmmm... not sure who wrote that but he forgot to point out that on the iMAC G4 800Mhz PPC (table lamp version) it DOESN'T WORK with 10.5...
I've tried it myself on my fully expanded iMACG4 and yes you can install 10.5 but...
Expect countless "Application Not Responding" crashes & lockups...
Random and totally unpredictable and unexpected Freezes...
Utils & Apps that you download written for OSX 10.5 not to install and most will (if they do install) quit with random errors telling you all sorts of weird stuff...
Ended up having to reformat the drive and doing a total re-install of OSX 10.4 again...
Definetaly not worth even attempting to install 10.5 on these particular iMAC G4 Models as you'll soon realise if you do that it doesn't have the power or the GFX Board to cope with 10.5 apps...
Erm... Frank, that page is a listing of machines that DO run 10.5 when you use the Leopard Assist program linked at the top of that page.
It runs on lowly 450MHz G4 Cubes faster than 10.4 does. It's more stable too.
1. Install 10.4
2. Download this file and unzip it
3. Run LeopardAssist.app (Click Options and then Optimize for machines with crap graphics cards)
4. Insert 10.5 disc and continue.
5. Profit!