OK here are some more comparison charts:
http://lowendmac.com/macdan/05/0202.html
http://docs.info.apple.com/article.html?artnum=58669
Bringing G3 iMacs and Other G3 Macs into the Tiger Age
http://lowendmac.com/macdan/md07/1207.html
There are some CD versions of tiger, but they are hard to find. You could buy an external DVD drive. It needs to be firewire and bootable by Mac OS. If you have another PPC with firewire, you could try target Disk mode and installed tiger that way.
You need to figure out the level of your firmware before installing 10.2 or greater. ( The PC name for firmware is BIOS. ) Installing 10.2 with a down level firmware will most likely make your iMac unusable and difficult to fix.
Figuring out what level of 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+of
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.
Some machine may not contain the firmware level but some other number which can be translated to the firmware level.
On my machine, I have 4.1.9f1.
What firmware do you need?
http://docs.info.apple.com/article.html?artnum=86117
For tiger, you need 256meg of memory. For panther, you need 128 meg of memory.