To know the difference between the two, take a photograph from a 640x480 cam and one from a 1 Megapixel cam and look at them on your computer. The quality difference will be quite apparent. Image resolution is definitely dependent upon the type of camera. Why do you think there are different MegaPixel values for digicams?
By the way, VGA stands for Video Graphics Array and not Adapter. Furthermore, 640x480 = 0.3MP and not 0.5MP.