Canon 20D has a
8.0MP APS-C (22.5 x 15 mm ) sized CMOS sensor . On the other hand, Canon D30 has a
3.0MP APS-C (22.7 x 15.1 mm ) sized CMOS sensor .
Canon 20D's sensor provides 5MP more than Canon D30's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the 20D and D30 sensor size comparison.
Canon 20D and Canon D30 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.