I probably will sound like a geek for saying this, but I have been planning on running a computer in the car. The display is a 7" touch screen because it fits very well into the radio hole in the dash. I've seen pictures somewhere of another person who did the same thing to a WRX. In terms of cost, it is in line with a deck that can do MP3s and possibly DVD playback.
Not having CDs strapped to a sun visor will be worth it to me. I figure with a wireless network card, I can make it automatically sync all new MP3s.
Of course if your not computer savy, this probably isn't a good option as it will require a lot of customization.