As a high school history teacher once told me, he knew he was in Berkeley when he looked up and saw a store called "Rasputin's". The real guy might have been creepy, but the music store is awesome (and don't forget to pick up a t-shirt with everyone's favorite hard-to-kill historical figure on it!). Huge selection of used and new CDs, vinyl, videos and more in every genre you can think of. Study hard before you go - the website offers reviews written by the staff, a chance to win tickets to their live shows, and information about the store.
Also, buy new CDs, or even sell your old ones, and Rasputin's will give you a voucher to take to your favorite school. Wait, you can make money selling your used music AND give back to the kids...yeah, this place is cool.