Olympus E-500 has a 8.0MP Four Thirds (17.3 x 13 mm ) sized CCD sensor . On the other hand, Olympus E-510 has a 10.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
Below you can see the E-500 and E-510 sensor size comparison.
Sensor Size and Resolution Comparison image of Olympus E-500 and Olympus E-510 Cameras
Olympus E-500 and Olympus E-510 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-500 has 24% larger pixel area compared to Olympus E-510, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.