Olympus E-500 has a
8.0MP Four Thirds (17.3 x 13 mm ) sized CCD sensor . On the other hand, Olympus E-1 has a
5.0MP Four Thirds (17.3 x 13 mm ) sized CCD sensor .
Below you can see the E-500 and E-1 sensor size comparison.
Olympus E-500 and Olympus E-1 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Olympus E-1 has 62% larger pixel area (45.71µm2 vs 28.15µm2) compared to Olympus E-500, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.