These are the steps I followed to update my installation to 10.5.7, I have no use for 10.5.8 because there is really nothing worth updating for in that version. Anyway this worked for me, so maybe it will work for you as well.

Credit goes to Imkantus for this great tutorial.

First of all I would always apply only the next higher update, so you can assess where problems occur.


1. Create a back-up of your working kernel using:
Code:
sudo cp /mach_kernel /working_kernel
Do the same with you Extensions folder, each time you update;
Code:
sudo cp -R /System/Library/Extensions /Extensions_bak
2. Make sure you have Disabler.kext and a Decrypter (dsmos.kext or AppleDecrypt.kext) installed.
3. Download the next Delta Update from Apple and apply it, but do not restart.
4. Open up Terminal and edit the first boot script using Code:
sudo nano /System/InstallAtStartup/scripts/1
go to the end and replace "Dont Steal Mac OS X.kext" with your Decrypter (2). *
5. Boot up using Code:
/working_kernel -v
for the next time.
6. Reapply system.kext and eventually even seatbelt.kext and AppleSMBIOS.kext from your back-up to avoid the known problems.

* This is no longer necessary at Update from 10.5.7 to 10.5.8 - instead of it make sure to repair file-permissions using OSx86 Tools before you reboot.

Report your problems and errors and I am sure we'll find a solution, feel free to ask questions about my offhand-written How-To

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OS : Snow Leopard 10.6.8
Motherboard : Gigabyte GA-M68MT-S2
CPU : AMD Athlon II X4 645, 3.11 GHz
RAM : 4GB Crucial PC3/16000 DDR3
GPU : nVidia Geforce 210 1 Gig Full Hardware Acceleration
Sound : Onboard Audio Realtek ALC887
LAN: Sonnet Gigabit PCIe Ethernet Controller
Monitor : Acer S231HL 24" Wide Screen 1920 x 1080 @ 60 Htz
USB: Keyboard/Microsoft Confort Curve 2000 - Mouse/Logitech M185
Bluetooth: Apple Magic Tracpad/ DLink DBT 122 Bluetooth Dongle