I used Carbon Cloner to clone the HD to another USB drive. Although it wasn't bootable, it was a complete copy including my user profile, apps etc. So if it went horribly wrong I had a backup.
This time, I removed the HD and put into a SATA / USB case. I booted from it, and it booted fine. Then I updated to 10.5.8. And then restarted.
Well it started OK, but, instead of showing the login screen, the screen went weird all scrambled.
So this is why the drive is in an external case!
Shutdown the Wind, connect the drive to another Mac, and remove some items from /system/library/extensions:
rename extensions.kext, restart and off we go.
Checking it seems that Quartz Extreme is not working... don't remember if it was before!
Update: the fix for Quartz Extreme is to re-install CyberGreg's drivers. Google for "MSi OSX86 Upgrade Pack v1.5".
Update 2: I also had the sleep problem. Found "SleepEnabler.kext", installed via kext helper.