The D5200 combines the D5100's swivel screen with the D3200's 24Mpix sensor and the D7000's light and focus metering systems. This gives it a max still-frame resolution of 6000x4000 on a 14-bit APS-C sensor (1.5x crop factor).

It remains to be seen, however, whether boosting the D5100's 16Mpix sensor to the D5200's 24Mpix will boost high-ISO noise and reduce dynamic range as well. DPReview's RAW noise tests show the 24Mpix D3200 to be noticeably noisier than the D5100:

http://www.dpreview.com/reviews/nikond5100/12

The D5200's new high-resolution sensor should definitely have an effect on its video image quality as well. If that turns out to be a negative effect, particularly in aliasing artifacts, the inevitable price reductions on the D5100 will make it a terrific bargain for a 14-bit APS-C hybrid stills/video camera.