Nikon D1X has a
5.0MP APS-C (23.7 x 15.5 mm ) sized CCD sensor . On the other hand, Nikon D90 has a
12.0MP APS-C (23.6 x 15.8 mm ) sized CMOS sensor .
Nikon D90's sensor provides 7MP more than Nikon D1X'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 D1X and D90 sensor size comparison.
Nikon D1X and Nikon D90 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.