I had prepared my early 2006 Intel iMac adding a 500G firewire/USB2.0 external MiniStackv3 to use with TimeMachine. I did a backup with SuperDuper prior to installing Leopard. It took quite awhile, but completed without incident.
I did a similar installation on a G5 iMac, also without...
I used a 22" Cinema display for over 18 months connected to a G4 533 Dual. I am completely spoiled and am saving up to purchase one to use at home, replacing my 19" Sony.
I had absolutely no problems and it didn't get knocked over during our last major earthquake, which knocked my 17" PC...