Try this method i posted earlier : http://www.infinitemac.com/f5/how-to...-5-6-to-t2980/

But remember, install voodoo 9.5.0 kernel ( system.kext will install along ) before it ( if you dont use it atm of course ). Restart your computer, and then begin the update as described in attached thread. Take notice that 10.5.5 introduced graphics updates do you'll have to delete/install anew all your gfx kexts after the update. I have made an update like this from 10.5.4 to 10.5.6, and then from 10.5.6 to 10.5.7 and it worked with no problems, so it should work here as well.

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