Cross your finger and hope the machine already has the correct firmware install. I could have a copy of the firmware. Let me know if you want me to search around.
Figuring out what level of Open Firmware you have?
1) Mac OS 9.x or 8.x, you need to use the Apple System Profiler.
Apple -> Apple System Profiler
2) Mac OS X, use the System Profiler.
Apple -> About This Mac
click on the More Info... tab
click on Hardware
read the Boot ROM Version
3) Open Firmware, boot into Open Firmware.
Power on your iMac while holding down command+option+o+f
The first output line contains the firmware level. Mine reads:
Apple PowerMac4,1 4.1.9f1 BootRom built on 09/14/01 at 13.18.04
Copyright 1994-2001 Apple Computer Inc.
On my machine, I have 4.1.9f1.
What firmware do you need?
http://support.apple.com/kb/HT1395
You can download the firmware from this page:
http://support.apple.com/kb/DL1283
filename: N-iMac_FWUpdate_4.1.9.smi.bin
avoid expanding the file on the download computer.
For a slot loading iMac, this article indicates that you need to be running 9.1 or later version of Mac OS Classic.
http://support.apple.com/kb/DL1283
"The iMac Firmware Update 4.1.9 will only run on iMac computers with slot-loading CD or DVD drives running Mac OS 9.1 or later from a local drive. If you are using Mac OS X you must boot from a local Mac OS 9.1 or later writeable partition (not a CD, or network disk) prior to following the update instructions."
You can download the Mac OS 9 updates from the Apple site.
http://support.apple.com/kb/HT1387?viewlocale=en_US
For tiger, you need 256meg of memory. A slot loader will take 1gig of memory.
You may need to get more memory.
get the 512meg card(s)
http://eshop.macsales.com/shop/apple/memory/iMac/G3_450MHz_500MHz_600MHz_700MHz
Mac OS X 10.4: System requirements
http://support.apple.com/kb/HT1514
PPC 10.4.11 combo update.
http://support.apple.com/kb/dl170