Each version of iChat and it's Updated versions, such as iChat 5.0.3, are OS specific.
With the exception of the first issue of iChat 2 which will work in Panther and the last Update of the OS before that each matches the OS as it goes.
iChat 3 only works in Tiger. By the time the OS is updated to 10.4.11 iChat has reached 3.1.9
It is similar in Leopard where the OS reaches 10.5.8 and iChat reaches 4.0.9 (or 4.0.8 depending where you look at the app) iChat 4 will not work on Tiger.
iChat 5 is Intel only and certainly cannot be installed on a G4 or G5 computer.
At this point the Frameworks include IMCore and some of the support achitecture is moved about.
In fact no matter which OS level you are at if you Reinstall iChat you have to Also Run the COMBO version of the Last OS Uppdate you did to Update iChat.
TO be clear, if you have a DVD Install Disk for Snow Leopard issued when it came out, the iChat version on there will be the base iChat 5. IF you have since updated to 10.5.8 then iChat will have been updated but the reinstall will have "Backdated" it to version 5. Software Update keeps receipts of what it has done so you manually have to run the Combo to Update the reinstalled iChat
It goes without saying that iChat 6 cannot be installed on Snow Leopard. (I also tried so I know)
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb( 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously