Canon 400D has a 10.0MP APS-C (22.2 x 14.8 mm ) sized CMOS sensor . On the other hand, Canon SD990 IS has a 15.0MP 1/1.7-inch (7.44 x 5.58 mm ) sized CCD sensor .
Canon SD990 IS's sensor provides 5MP more than Canon 400D'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 400D and SD990 IS sensor size comparison.
As seen above, Canon 400D has a 7.9x Larger sensor area than Canon SD990 IS. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.