Olympus E-1 has a
5.0MP Four Thirds (17.3 x 13 mm ) sized CCD sensor . On the other hand, Olympus E-300 has a
8.0MP Four Thirds (17.3 x 13 mm ) sized CCD sensor .
Below you can see the E-1 and E-300 sensor size comparison.
Olympus E-1 and Olympus E-300 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 compared to Olympus E-300, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.