Well, you would probably be able to shoot more frames in a shorter timespan (FPS), the images might be a little bit better because of a better sensor, it might or might not be weather-sealed, might have some extra features, it all depends on how much you upgrade. Good lenses basically increase image quality and give you a larger aperture (means more out of focus backgrounds & a faster shutter speed), possibly increase your focal length (or decrease it in a wide-angle). But they do their job VERY well if you spend enough cash.
Also, sorry for not replying sooner. I was just going through my old posts.