I'm on an AMD w/ Voodoo 9.5 Kernel and I am posting from 10.5.6. I downloaded and installed the Universal OSX86 Installer and selected Install PC_EFI v9. After that, I downloaded and used the DSDT Patcher GUI and selected all the options. After that, I backed up some kexts and data to be safe and then got the 10.5.6 Update from Software Update (not the ASU; I got an error every time I tried).

After restart, it booted! I had to replace AppleSMBIOS.kext with my old kext because About This Mac would force a logout, but that all I needed to do to get 10.5.6 working for me