Can u link me with those boards? cause i would like to yea maybe purchase one and get some 10.4 on my imac
As another poster has already put up some links to boards, I won't.
The logic boards can be swapped after removing the bottom plastic and the metal cage underneath it. You'll be able to get at the drives, memory, and Airport slot easily too.
You need to knock holes out or mod for the Firewire and ext VGA because the older slot loads don't have the openings for these. This is fairly easy to do.
If you're not familiar with CRTs or analog TV tubes be very careful round the iMac CRT. It can potentially hit you with kilovolts without being discharged before hand. But the good news is that a logic board swap alone doesn't involve this. Just make sure the power is disconnected, not just shut off.