Since you have a 333MHz iMac which doesn't have a VGA out port, this will be even more difficult than it was for me. The one I worked like the one iphil has, includes a vga port on the outside of the case. What happens is that when you try to install OS X without doing the firmware update, it fakes the computer into thinking that the filter board is bad. If you call Apple, or take it in to get repaired, they will tell you its bad when actually its not. The key is to get the firmware update installed. Since you're a Mac user who sounds like you've been using Macs for quite a while, you saw that the display didn't turn on, and thus reset the PRAM. Actually, all you did was make things 10x worse by doing that. When I did when I did that is I took the RAM out, unplugged the computer, pressed the CUDA reset switch ONCE which resets the power going into the board and I was able to get the computer to try and boot, and it would boot, but anytime I had to restart I would have to fool around trying to get the computer to boot again. Thats why this is a time consuming process. I was able to use an external display to get OS 9 installed and do the firmware update. The internal display won't kick in until you do the firmware update. This is why you need the external VGA port.
What may (notice "may") work for you, is if you can get ahold of an old Macintosh Display with the old style display connector. I think thats either a 14 or 15 pin connector and will kinda look like the male end of a parallel cable with only 14 or 15 pins in 2 rows only unlike a VGA cable which has 3 rows. When you go to pull the logicboard assembly out of the 333 MHz iMac, you have to take the display connector off and you'll see the connector and what it looks like. So if you can get a display that has that connector on it, and can make it boot from an OS 9 CD then erase the HD and reinstall OS 9.1 (bare min OS for the firmware update), do the firmware update, and then restart the iMac into OS 9 one more time, then shut it down, unplug the external display, and then plug the internal 15" display back in and it should work.
Like I said in my first post, this is a huge pain in the @$$ to get working. Its a wonder Apple didn't get sued over this and a ton of people did the very same thing you did when OS X came out. They did get sued over some G3 processors not working with OS X, but it had nothing to do with this particular problem.
Let me know if you have any problems. You can even use the email address I've provided for iphil if you want.