the build b1gapl suggested will be suffice for u and it will last u as long as u want it to last. i dont think u will require more than 320gb for hard drive coz ur just downloading music. and later if u need more, u can just buy another one when u feel the need, coz hard drives are going down in prices, so u'll save a lot when u buy another one later when u need it. but i would suggest 2 gigs of ram right now so u wont need to upgrade anytime soon.
good luck