disable isight or use portable??

Is there a way to disable the built-in isight camera to allow the use of either another isight on a cable or quickcam on a cable? I need to be able to aim it to other parts of the room and around to other people when making short clips for email and it is next to impossible to have everyone run around in front of the computer or pick up the whole iMac to aim it to other parts of the room while putting on small skits and such. So, I was wondering if I went and bought another isight from the apple store would it allow me to turn off the built in and use the regular one instead? I can find no way to make it turn off so I can use my Quickcam. My quickcam works fine on my Powerbook and iBook, but the iMac wont r3ecognize it because any video app I turn on automatically grabs the built-in isight instead and has no way to turn it off. The built-in is great for chats, but very impractical for anything else that I like to use a webcam for. Help!

Looking in the manual, it looks like there isn't a way to turn it off. Seems like you should be able to connect a real iSight though. I've seen it done on iMacs before.

Do you know anyone who has done this?