Sorry to be uninspiring, but I'm on a budget of nearly $0. In retrospect, I wouldn't choose a desktop PC again - the power issues are too complicated. Buying a display that fits in a standard DIN slot should make things prettier and easier... especially if you don't do it yourself :P. Anyway, here's another one:
http://www.mp3car.com/vbulletin/show...t-install.html
And I know of someone else who has a CarPC in an Elantra but I haven't seen him on this forum. He goes by "sed" in ElantraClub.com
Good luck