I investigate this a while back and the answer is that computer create too much interface for AM, plus FM reception probably isn't that great with the cards. The answer is to get an external unit (scanner/radio) and somehow interface it via USB/serial/etc....
here is a link to help you get started in that direction. i am still in the early stages of my project so i haven't got to this stage yet.
Option to is to do some reasearch about the upcoming Satellite Radio being launched this summer and connect that receiver to your pc.