I eventually had to re-install 10.4.7 on the latest iMac (which works, finally) and had the same issues with Photoshop CS and the Microsoft Office suite that I mentioned previously.
I did a little digging, and this is only an issue for Intel-based machines. I also read on a forum somewhere that Apple had posted a patched version of the install. I went to the Apple website and downloaded this 10.4.7 update and reinstalled it over the previous 10.4.7 version.
Apart from a weird glitch late yesterday arvo, this seems to have fixed the problem. Just thought I’d share the solution in case someone else comes across this issue.