This all depends on what you use for a bootloader... however, there is an easy way to ensure the voodoo kernel you are using won't get overwritten with a native apple kernel: just rename kernel to something (like mach_kernel.voodoo) then add the renamed kernel to your boot flags in your com.apple.boot.plist file. I would make sure you replace the seatbelt.kext and System.kext files if you get errors during normal use but otherwise if your system was configured for 10.5.6, it should work with 10.5.7 as well.

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