Olympus E-420 has a 10.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic III processor. On the other hand, Olympus E-620 has a 12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic III+ processor.
Below you can see the E-420 and E-620 sensor size comparison.
Olympus E-420 and Olympus E-620 have 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-420 has 22% larger pixel area compared to Olympus E-620, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.