EOS Technology Package
We had the technology package on ours when we got it and really, can't live without it. I hardly listen to the radio again since a 20GB hard drive for music is more than enough.
Tip: move all your music into one folder and then create a folder for each of your playlists and then move them in there as you find songs you like. We made the mistake of moving them into individual folders first and are now moving things around the hard way.
I was skeptical at first - but it definitely was a good decision.