Nikon D80 has a 10.0MP APS-C (23.6 x 15.8 mm ) sized CCD sensor . On the other hand, Samsung NX300M has a 20.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor and features DRIMe IV processor.
Samsung NX300M's sensor provides 10MP more than Nikon D80's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the D80 and NX300M sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon D80 and Samsung NX300M Cameras
Nikon D80 and Samsung NX300M have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.