Both Nikon D80 and Olympus E-400 have 10.0 MP resolution sensors but Nikon D80's sensor is APS-C (23.6 x 15.8 mm ) and Olympus E-400's sensor is Four Thirds (17.3 x 13 mm ).
Since Nikon D80'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 Olympus E-400.
Below you can see the D80 and E-400 sensor size comparison.
As seen above, Nikon D80 has a 1.7x Larger sensor area than Olympus E-400. 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.