Installed 10.5.7 last night with tips from this thread. Went perfectly.
One tip, if you are having trouble getting to the boot prompt to enter -x, or just dont know what that means, or arent sure you're getting it right...
you can substitute holding down shift from power on/reboot until at the safe mode login screen (where it asks for your password). holding down shift = safe mode, and passing -x as a boot flag = safe mode. i believe they are equivalent.
This means hold down shift a looooong time, your finger might get tired

but it works fine. I actually held it down from the initial restart all the way through the double reboot to the login screen. not sure if thats really needed but it works