Just yesterday I solved a similar problem on this model PowerBook for a client.
After the startup chime the PowerBook would essentially appear to be dead, did all the steps above but nothing helped until I took out the RAM:, only one RAM stick worked, and in only one slot too, but this didn't solve the problem in its entirety.
After playing with the RAM I was able to start up in Open Firmware by holding the Command+Option+O+F keys down after the startup chime upon reboot, and reset the NVRAM by using the following commands (press return after each line, it will reboot after the last command is entered)
reset-nvram
set-defaults
reset-all
After doing the above the computer rebooted and I got the blinking folder with a ? as the computer searched for a system to boot from, okay- lets try booting in Safe Mode.
That didn't work and the computer went right back into Open Firmware, did this a couple times, so next time Open Firmware was up I typed in mac-boot
but pressed the Option key immediately after pressing the enter key in Open Firmware, this brought up the screen to choose a startup folder/disc, but it didn't show anything, so I connected an external startup drive which wouldn't show up either (I rebooted as above a couple times and it wouldn't recognize any drive connected).
Okay... so after a little googling around I found that perhaps the Airport card was causing a problem.
I took the case apart, unplugged the Airport card from the logic board, put the case back together, crossed my fingers, pressed the start button, heard the chime.... and the PowerBook started up like nothing had ever been wrong.
So there you have it, in my case (besides the RAM) the problem was a bad Airport Extreme card. I currently have the ribbon cable for it plugged back into the logic board, and it still boots, but with no Airport card recognized.
If the client wants an Airport card put in I'll take it apart further, put it in, and see if it works with one on Monday.
I hope this helps you, or someone else down the road.
A note however, the one RAM slot still doesn't work, and I seem to recall there was an issue with a PowerBook not sure which model, where a problem would arise in which there was insufficient power going to the RAM slots.