Harddrives in the head unit aren't the way to go I hear. For one, they're terribly expensive. There are tons of external units, some of which you can load up with mp3'* from your computer via wireless networking. You know...I was just browsing crtuchfield and didn't see any units w/hd'* on sale anymore...
My friend and I looked into making an external HD unit. Then we realized it'd be ten times easier and cheaper to get a portable mp3 player (like the muvo2) and load it up with 4 gigs of music and feed it through an external input. Plus, you can hold it in your hand, like a wired remote, making things way easier, and the display is nice and big.