for higher iso, the sensors try to response the exposure, specially in low lights, as fast as possible, that gets more noise. you can take 8MB images and downsize them using bi-cubic method, it would filter some noise away.

if you take pictures in very bright lights, but not get enough exposure, you can get lots of noise also.

I don't really the sensor electronics. but, for canon S5, you get as good as it gets. there is no that much you can do.

your camera is pretty heavy: 550 g (19.4 oz)?