HI Jayster,
http://support.apple.com/kb/HT1159?viewlocale=en_US lists the OS builds shipped with the intel apple computers. The current imacs use OS Builds 10D2322a to 10F2108. The original 20" core duo uses an earlier build; and such cloning the drive would most likely cause issues.
http://support.apple.com/kb/HT2186 explains in more detail.
"If you install an earlier (previous) version of the Mac OS X than what was included, your computer may exhibit unexpected behavior such as:
The trackpad or mouse may not respond properly
The computer may stop responding
Sleep/wake issues may occur
The display image may appear to "shrink" with black bars around it, may appear tinted, or have other issues
Loss of built-in audio
Loss of Bluetooth or AirPort functionality
May not start up past the Apple logo"
If your time machine back up is up to date. The least amount of work, would be to use Migration Assistant to migrate your time machine back up to the new computer.
You could also use Migration Assistant with the original hard drive in a external enclosure. Migration assistant would take less time to migrate the data. But taking the computer apart sounds like a huge pain.
Did you try using disk mode? It's allot easer then taking the computer apart.
I hope the links answer all your questions.