Old USB 1.1 ports-devices will only show "Speed: 12Mb/sec" as thier maximum bandwidth. Modern USB 2.0 high-resolution UVC webcams utterly overwhelm USB 1.1 ports which are more than FOURTY TIMES SLOWER.
Some cams won't display anything - or if they do you'll find your Frame-Rate is so abysmally low you'll just end up in sluggish, slow-motion frustration.
Apple intentionally does not allow external USB Mac webcams to be selected as a video source in i Move. Try a special hack from Mactaris that allows i Move to 'see' an external 3rd-party web camera.
With older versions, you CAN use an external UVC cam as it's source *IF* you put the built in i Sight 'In Use' first.
Launch i Chat, open your Video Preview window to put the i Sight in-use. Simply Put: Your webcam may have a very very hi-res HD sensor.
Start in your Apple Address Book: Family & friends with, say, @or @are good candidates for Apple Messages, i Chat, @for Yahoo Messenger, @or @for MSN Messenger buddies.
Or try a video session with Face Book friends online.
Most Quick Time compatible apps allow selecting and switching between multiple video sources on the fly; Nice and Civil to Down 'n' Dirty.