Cardio, not crunches, will make your belly flat.  Generally, muscle is contained under a layer of fat, so the only way to be able to see that muscle is to get rid of the fat covering it.  Aim for at least 30 minutes of cardio at least 3 days a week for a flatter, firmer stomach.  Also, don’t forget to do your crunches to build up the muscle, but you don’t need to do hundreds of them.