Patrice
No longer a newbie, moving up!
There is a monumental difference in your ability to crop a photo taken with the D5200's 24 megapixel sensor and the D5100's 16 mp sensor - I know for a fact because I've owned and shot both cameras. I can't even imagine how much difference you'd see between the D5200 and the 12 mp sensor of the D5000 when you go to crop. The differences there would be mind boggling I would think.
Monumental? Mind boggling?
Not to be too pedantic but here are the numbers:
From 16 to 24 mp is roughly a 21% increase in the linear dimensions and from 12 to 24 is roughly 42%
X x 1.5X = 12, x = sqrt of 8, x = 2.8, so a 12 mp nikon is 2.8 million pixels by 4.2 million pixels
same calc for a 16mp nikon gives a sensor of 3.3 by 4.9, hence the 21% dimension increase and 24 mp camera has a 4 x 6 million pixel sensor thus a 42% increase from 12 mp.