10.6.7 Fine 10.6.8 Unbootable ?!?!?!

Question

Fellow Mac-users,
Since a few days I have trouble with my iMac 20" (early 2009). I was running 10.6.7 without trouble but when I run the 10.6.8 update it won't boot anymore.
When it starts up the screen stays grey. It's either plain grey or it's a grey screen with black dots covering it. I've tried everything I could think of so far and kept a list of my efforts. I did all this and actions mentioned below without any equipment attached, besides keyboard and mouse.
I've managed to format the HD from an external HD containing OSX 10.6.0, this way I can keep trying different OSX's.
After re-installing OSX in the following orders without a result I am fresh out of ideas.
- Format HD, install 10.6.0 then update to 10.6.7, which works. As soon as I update to 10.6.8 it hangs again(grey screen). I've tried the update from 10.6.7 to 10.6.8 as well
  as the combo pack from 10.6.0 to 10.6.8
- Format HD, install 10.6.0 then update to 10.6.7, which works. Then tried to install 10.7 (Lion) and it doesn't boot either.(Gray screen)
- Formatted HD partitioned in 10.6.7 times 2. I can run both, but as soon as it's updated to 10.6.8 or 10.7 I can't boot that partition anymore, it goes grey after
  the Apple-logo, which can be seen in the display but without the thinking/spinning wheel. Though the one with 10.6.7 is still OK.
The list below summarizes other actions taken so far, besides various manners of installing OS's before trying to run 10.6.8 update:
- Started up in every modus possible, including resetting video memory.
- Restored/checked disk permissions after/between every action.
- Checked HD with Apple Utility programs, Diskwarrior and Drive Genius 3, harddisk seems to be in order every way I try.
- Switched RAM modules with other iMac (neighbour's, same build and age), RAM seems fine as well.
I hope someone has the answer. Thanks for reading.
Paul

Answer 1

The iMac didn't need a firmware update so it said but after some searching we found out the kernel had some problems booting.
We managed to fix the PCI-configuration by replacing two kernel extension files.
We used the 'Kext Wizard' application to replace the two following files:
AppleACPIPlatform.kext
IOPCIFamily.kext
After this the PCI-configuration went as it should and we were able to install 10.6.8.
A new problem has risen now that we finally installed 10.7.2 but the iMac doesn't see the airport-card. After some reading we concluded that the best way to proceed is to restore the Boot-ROM using a restore CD for that model iMac (8.1) (EARLY 2008). Different then earlier written this post.
Does anyone know where to download a restore CD for iMac 8.1???