Ok, a couple of quick thoughts here as an owner of both a 16 mp D5100 and then later on a 24 mp D5200. Yes, there is a noticeable increase in picture/image quality between the two when two shots are taken with each under controlled conditions. Please note, it's noticeable. It is not earth shattering, mind numbing, or awe inspiring. But it is noticeable. Please also note that generally your not shooting under controlled conditions and your final IQ will be influenced a lot more by lighting conditions, technique, etc.
The real big difference between the 16 mp and 24 mp sensor is the final image size - the larger image size of the 24 mp sensor gives you a lot more options when you go to crop or post process a photo, and this is extremely useful if you shoot a lot of telephoto (like I do). On shots that don't require as much cropping/resizing, the differences will not be as noticeable or appreciated.
On the 3x series and 5x series cameras you will want G series lenses. These cameras do not have a built in autofocus motor and the older D models lenses that lack a motor built into the lens will require you to manually focus the lens. Not something I generally recommend unless you only want to shoot at stuff that doesn't move and you always have plenty of time to get the lens adjusted and focused before you take the picture.