I'm really not sure about AGP, but I have had Ubuntu/Debian running on my MacMini G4 without the problem you describe. The MacMini has a Radeon card.
I think solving your problem could help a lot of people, and then we could improve the wiki too. When the screen freezes, can you get to a command prompt using ctrl-alt-f1? Have you tried "video=ofonly" at the boot: prompt before startup? If you can get to a command line before everything freezes you might also be able to try the "fbdev" driver for XWindows that often works when other XWindows drivers fail.
This post might interest you: http://www.ppclinux.info/boards/1/topics/show/38
If none of those ideas get you anywhere, can you please repost your request in the main forum? It is much more likely to get attention from someone who has the same machine as you if it has a useful subject line.