Both Nikon D3 and Nikon D2X have 12.0 MP resolution sensors but Nikon D3's sensor is Full frame (36 x 23.9 mm ) and Nikon D2X's sensor is APS-C (23.7 x 15.7 mm ).
Since Nikon D3's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Nikon D2X.
Below you can see the D3 and D2X sensor size comparison.
As seen above, Nikon D3 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
Both Nikon D3 and Nikon D2X sensors have been tested by DxoMark. DxoMark 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. Of the two cameras that we are comparing, D3 has scored 81, 22 points higher than D2X.