I recommend Cars.com, personally. I've checked sites like Edmunds, MSN Autos, Yahoo! Autos, and Cars.com is the only site that lists the packages for used cars in a very clear and usually accurate manner.
Here's a link to the options page for the car
But just to answer the question, if you get the upgraded sound system (Premium package includes this), you'll get a Bose 11 speaker system with a 6CD changer. If you get the Navigation package (also includes the upgraded sound system), you also get a 9.3 Gig section of the Nav's HD as a music storage section. I think if you go without Nav, but get the Premium package anyway, you still get a 2 Gig section for that, but I'm not positive.