Actually the 400Meg iMacs are much easier to install HDs and CD drives than the tray loaders. That said should you decide to try a swap; shut it down and unplug it for a couple of days. No way do you want the full voltage from the CRT zapping through your sweaty palms to ground.
Hopefully someone can post a link. If not I will search my archives and see if I still have directions for this.
One other thought; be sure the you are connecting the USB to the port on the computer not the keyboard. The keyboard is essentially an unpowered port and some devices simply do not work properly from that port.