Nikon D2X has a 12.0MP APS-C (23.7 x 15.7 mm ) sized CMOS sensor . On the other hand, Nikon D3X has a 25.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed processor.
Nikon D3X's sensor provides 13MP more than Nikon D2X'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 D2X and D3X sensor size comparison.
As seen above, Nikon D3X has a 2.3x Larger sensor area than Nikon D2X. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Nikon D2X and Nikon D3X sensors have been tested by DxO and the results show that D3X has a better overall score of 88, 29 points higher compared to D2X's score of 59.