If you will use it mainly for Yahoo, then consider a webcam that will sit nicely on your PC or monitor. Sounds simple, but if it hasn't got a nice base, or is hard to point and keep in one place, it'll be a pain.
Specs? Well, it should have a reasonable frame rate (say 20 to 30 fps) and a good max resolution (say 640 by 480). Built in microphone may be useful for messaging.
A review of the Logitech QuickCam 3000 is
here. My family uses one of these, and it's great for messaging.
Depends what your price range (budget) is as well.
Cheers
Mick