AVR-Chat

with all the talk about usb here, i have wanted to make an mp3 player
that interfaces via usb to the computer to download songs to it.
either this or a usb key (i'd say this is easier :D ). 
at http://www.myplace.nu/mp3/yampp4.htm their mp3 is recognised at a
drive. is it that simple that all i need to to is add a usb chip or do
i need to write code to get it to work like this?
also any sites/help on making something like this would be great too.

thanks for any help
jamie