spamdies,

Great guidance by those that posted before me. To answer the question posed by your thread title:

how much is really necessary for the home pc
The degree of security you establish on your home computer should be based on what you are attempting to reasonably protect. Keeping in mind, there is no absolute security, but your hopes are to make it as hard on the deviants as you possibly can so they will look elsewhere.

Additionally you must be just as responsible for what leaves your computer as you are for what enters. Meaning you do not want your private, financial, etc., information to land in the hands of a criminal and you do not want to become a hop for illegal activities.

Employing the recommendations listed in the other posts above will provide you with a reasonable amount of security for the home pc.

cheers