low iso is to reduce the noise level. Some people say it is feasible to increase iso to reduce the exposure time so that hotpixels won't be so jialat, but in my experience, the increase in noise due to ISO is not worth the reduction in hotpixels due to lesser exposure time.
Aperture should not affect the graininess, but will affect sharpness, because of DOF. Using the smallest aperture will cause more of your subjects to be sharp, but will cause exposure to be longer.
Perhaps the graininess is due to your cam? What cam is it, and what kind of subject in what lighting conditions are you trying to capture?
I think for digital cam, both ways you lose. You just have to settle for the lesser of the two evils. The first evil is high ISO noise, the second evil is long exposure noise. So, experiment until you're satisfied and stick to that type of settings.
For my G1, I prefer long exposure noise. Anything above ISO100 on the G1 is ugly.