I had a somewhat similar experience, except I wasn't able to finish the install. I was moving along through the installer pretty well, but then the colors on the screen inverted and the mouse froze. I reset, but got a folder with a broken computer on it (not the flashing question mark). I eventually had to do a hardware reset in Open Firmware and pull the PRAM battery from the motherboard for a few minutes before I could boot up and wipe the drive. (This is a G3 B&W 400 DVD, 192 MB RAM.)
I've tried installing Mandrake 8 PPC on several machines, and haven't succeeded on a single one of them. Maybe it's best to wait for the next update. The support just doesn't seem to be there yet.