I need a high quality EXTERNAL web cam - what specs should I look for on the box?
Mac Book Pro OSX Snow Leopard
Three things are important when choosing a webcam.
I noticed from your previous question that you have a 13" MacBook Pro. This is capable of displaying 720p resolution but someone on the other end of your video call may be capable of displaying 1080p or more. So the resolution you need depends on how you're going to use it. If you're making videos of yourself to be edited later you might like to choose a camera that's capable of 1080p.
You would need a camera with a minimum framerate of 30 frames per second (FPS) to get smooth video. Higher would be even better. However, be aware that some webcams will lower their resolution to obtain higher framerates so if keeping a high resolution is important you need to check what framerate the camera can sustain at each resolution.
Low-light quality is something that a lot of webcams are bad at. Manufacturers may make claims about their webcams which are not true so it's best to read reviews and try any webcams you're considering before you purchase.