regarding the crashes check that the translate file in /oah has not been patched to make sure copy it over from the dvd. Because if this is patched anything that has ppc code in it will just bomb on amd. When you say 10.5.5 makes the machine unbootable what happens is it graphics problem if so try to remove any ati graphics kexts. Because the last few updates from apple changed quie alot of the ati kexts. If its not this post the last message when booting with the -v flag