Image quality is really more dependent on optics than megapixels... My 300D with 6 megapixels will dust any 10 MP point and shoot... webcams are the same way. iSight looks good because it has better optics.
It's not an amazing quality webcam, but it's serviceable, the pictures aren't anywhere close to my ixus i (4 Megapixels), but it's noticeably better, at first glance even, than the camera on my Nokia 6230i which is 1.3 mega pixels
Erm... that would be the stuff in between you and the sensor. Like the lens. The integrated iSights appear to use plastic fixed-focus lenses, but somehow Apple does an outstanding job with them. I'm guessing they're not focused at infinity like camera phones are, and perhaps that's why? Given that you are sitting in front of the MacBook, etc, it's possible to know roughly where you will be and use a fixed focus....