If the webcam you have does not come with a mac driver, try the external webcam app called macam. I have used it to interface an external webcam with webcam enabled apps before so it may work for you.
Garagesale should have a preferences somewhere to select between cameras. If not, install the macam drivers, disable the built in iSight (Google should be able to help, I can't remember exactly what to do), and see if that works.