I figured I'd share my experience with a 500 MHz Titanium PowerBook. Installing Hardy is possible, but you'll need about 384 MB RAM at the very least.
The problem, in my case, was the video card. I have an ATI Technologies Inc Rage Mobility M3 AGP 2x (rev 02). When booting the Live CD (not the alternative CD, that is), the screen turns glitchy white and the system won't boot.
First, boot the system by typing live-nosplash vga=1 at the prompt.
The system will start booting and eventually end with a glitchy screen as X is being started. What we should do is go to a tty by pressing CTRL+ALT+F1 (or any other function key other than F7). We should reconfigure x.org, as it was not able to correctly determine our screen's settings.
Type sudo nano /etc/X11/xorg.conf to edit the configuration file. Since this is a Live CD, you don't need to worry about making a backup, though this is normally a good thing to do.
Now head over to the section "Device" and add:
Driver "ati"Then, in section "Monitor", add:
HorizSync 30-60(Those values were taken from saweso.com--thanks!)
Modeline "1152x768" 65 1152 1178 1314 1472 768 771 777 806 +HSync +VSync
In the section "Screen", we should add the following:
Now close the file by pressing CTRL+X and then choosing Y to save our modifications.
We're ready to start X now, but for some reason nothing happens when you try to do it as the "ubuntu" user, so instead we'll type sudo startx. It might tell you that the X server is already active; if that's the case, follow its instruction to remove /tmp/.X0-lock before trying again (sudo rm /tmp/.X0-lock will do the trick).
Finally, in order to install Ubuntu, just head over to /home/ubuntu/Desktop/ and click on the installer icon.