Nikon D80 has a 10.0MP APS-C (23.6 x 15.8 mm ) sized CCD sensor . On the other hand, Sigma DP2 Quattro has a 20.0MP APS-C (23.5 x 15.7 mm ) sized CMOS (Foveon X3) sensor and features TRUE III engine processor.
Sigma DP2 Quattro'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 DP2 Quattro sensor size comparison.
Nikon D80 and Sigma DP2 Quattro 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.