The computer is a bog standard G4 400 MHz Sawtooth in which I did drop in some slightly slower RAM (100 vice 133), but I don't think that would affect installation. Would it?
The Sawtooth 400 has 100 MHz FSB, so this should be no problem. However, since you took the RAM from somewhere, maybe it is faulty or the G4 has problems with the type of RAM (timings etc).
I'm suspecting a bad CD burn at this point in time.
Might be... try to burn in slowest mode possible. Furthermore, you might want to check the MD5 sum of the downloaded ISO.
