IMac won't boot, dark screen after normal grey start up screen

Quite perplexed with this problem, have tried everything I can think of.
Computer: iMac 2.16 gHz 24" mid 2006, 4 gig RAM
OS: 10.6.8
Froze and required hard shutdown.  Subsequent to this, has behaved consistently:
Normal grey screen with Apple logo and spinning timer, can hear disk activity. Then the screen goes dark and nothing else happens (although the computer is still on).  Have to  perform hard shutdown.
Have tried:
1. Booting into single user mode (multiple times) and performed fsck, no errors noted
2. after using 'exit' command in single user mode, computer attempts to reboot but then freezes after noting 'panic'
3. Reset PRAM (no change)
4. Tried safe mode boot, progress bar on grey screen progresses, then spinning timers comes on, then same crash with dark screen
4. Disassembled computer, removed hard drive, mounted normally in external SATA enclosure.  I backed up critical data, ran Disk Utility Repair, no errors noted.
5. Reinstalled HD, reassembled computer, rebooted, same issue
6. Attmepted to boot with Snow Leopard Install disc to perform a fresh Achive and Install - after prolonged grey screen with spinning timer and audible DVD drive access, same switch to dark screen and no further progress
7. Same panic message and stalled boot attempt noted when booting into single user mod
I'm stumped.  My assumption is that there is a courrpt file that is causing a kernel panic during boot and I assumed that I'd be able to perform an archive and install but when this failed I'm guessing all I can do is replace the hard drive with a new one and then go through the hassle of transferring all the data over manually.
I would welcome hearing any other observations, similar experiences or other ideas.  MTIA.

One update ... when I boot the computer and get the 'dark screen',  I hear disc activity.  So I started hitting the return key and I get the chime with each return as if I'm on the log in screen and hitting the return to log in without entering any text.
That would argue there is a problem with the screen.
However, it does not explain why the computer display works normally during single user mode and hangs (without the screen going dark) when trying to reboot from this mode.

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