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!
I did use LeopardAssist with the install and the results were as posted above, I also visited many MAC forums and almost everyone using this particular model of the iMACG4 had the same problems... (and other ones that I hadn't encountered yet)
People far more knowledgeable than me on these forums were explaining to others (like myself) that because of the type of GFX card, Bus speeds etc... that trying to run OSX 10.5 one this model is pretty pointless as most of the apps and utils written for 10.5 will as I've described either crash or will run so slowly and unresponsive as to make them unusable...
I've tried it myself and believe me unless all you want to do is run the OSX 10.4 version of Safari you regret ever installing 10.5 on an iMacG4 @ 800Mhz, (if you start to upgrade Safari to the 10.5 versions you end up with a browser that is so slow and unresponsive it makes the 10.4 version look super fast)...
If you don't believe me get yourself an old iMac G4 of the type mentioned and try it for yourself then you'll understand how bad it is. (heck even Apple themselves and their tech support will tell you the same)...