Honestly, cameras are not a necessity for streaming, but it seems that a majority of streamers do use them. If you are wanting to go down the face-cam route, it seems that the Logitech C920 is the one to go with. This camera offers 1080p video capture at 30 FPS and 720p at 60 FPS. It can also be mounted to your monitor or put on a tripod.
I personally haven't used this camera -- but after looking at reviews, it seems that this is the camera to buy.