iMac - How to switch from built-in to external USB camera?

Hi,

On my iMac (2GHz, 4 GB, OS 10.5.6) I'd like to switch from the internal (built-in iSight) camera to an external USB camera. The system recognizes both cameras in the USB the hardware profile... but I cant find in the system preferences how to activate the external camera or to toggle between the iSight and external camera or possibly even run them with 2 different applications (i.e. one application, a messenger with the iSight camera and simultaneously a streaming webcam application with the external usb camera)?

Any hint would be welcome.... thks
Vivifiant

Are there maybe drivers that came with the external camera? I'm sure you got a cd with the cam and there might be some additional software, maybe even help files

I myself have never used an external webcam, so this is just a random suggestion