Original iMac System Software Missing

[Jay Friedland]

I have a troubleshooting question with a few different parts-
1) For my son to use Warcraft III, we needed to add memory to a G3 
iMac, with 333mHz.
After a few attempts to load an additional 128 into either slot, we 
wound up with a blank screen.

2) After going back to only the single board of original memory, the 
iMac had the question mark file on opening.

After trying Disk Warrior and a bootable 9.1 CD, neither booted the 
computer. He is missing his System Software CD.
What's our next step to correct this? Is System Software CD the only 
way to get it to boot? How can it be replaced? Downloaded?
Anyway to tell what the original problem could be with the memory 
failing to start (PC100 and correct fit)? We actually tried 3 different 
boards with the same results.