I use a MacPro dual Quad Core 2.8 GHz with a 23" Viewsonic monitor and have a iSight Camera I took out of a 20" G5 iMac and spiced a USB cord onto it, (Matched color to color of cord) and use it for FaceTime and Skype. It even looks better than the old iSight external camera. I do have to use a Blue Snowflake USB Mic for the sound.
Try it as a usb plug in first before tearing the unit down to replace inside. There are a bunch of different configurations to the iMac cameras and most will not fit inside of the Cinema Display cabinet. Thats why I use it externally.