the display remaining in a sleep like state wile booting can be fixed. in open firmware its normlly set to use a different output thats no longer installed, in xpostfacto there is settings to choose what output (normally the monitor/video being used) you need to make sure that the proper output is selected. that should cure your booting with no video.
check in ASP in OS X and OS 9 to see if the system is seeing a ghost video card, sometimes the systems will see a ghost video card and OF will try to use that as the output for OF instead of the onboard. usually you can get into OF and reset it and it will start working again.
to get into OF press and hold command + option + O + F
type init-nvram (press enter/return) some later Rom versions its reset-nvram
type set-defaults (press enter/return)
type reset-all (press enter/return)
after every entry after you press enter you should see OK if it says unknown work or somethig to that matter (mostly with the init-nvram or reset-nvram comands)
that is one out of a few ways to reset, another is to hold the CUDA for a few seconds ( i normally hold for 15 - 30 seconds)
or you can go and buy a Radeon 7000 PCI and it will fix it.
and yea that link to the apple docs is the correct work around but however there is steps you need to take if your doing it in panther and tiger.
only do the terminal commands if you have a Mac PCI video card
in panther and tiger you need to install fresh and do the fix (terminal commands both sets) then reboot, you must have a second Mac compatible video card to use cause the onboard video will be software disabled and you will not see anything when you restart after you do the fix.
then with the PCI video card (the only screen output you will be able to use) you install all of the updates and reboot, and on the reboot the onboard video will be re-enabled and will not black out again.