Replacing your head unit will definitely provide an improvement in sound quality. Sadly, it will be just a matter of time until your factory speakers stop working as they will not last long with the increased power an aftermarket head unit will be providing to them. It will not give you a significant increase in bass response (this is most associated with the size and quality of the speakers as opposed to the head unit).
When you install the head unit, amplifier, and subwoofer, try to look into replacing your factory speakers to something that will handle the increased power your new head unit will be providing.