A DVD system with a 6.4" screen installed will run you around $750 at the wholesale level, and around a grand at the retail level.
A Compaq Presario laptop with a 14" screen, 1.6GHZ Athlon, DVD player, 256MB ram, WinXP, and a 20Gig Drive will run you around a grand. An extra $30 will get you a cigarette lighter 300 watt inverter.
I've got both (a ceiling mounted 6.4" vehicle system, and the laptop) and the choice is painfully clear to me.
(guess what I am typing on right now in my undies on the bed - oh yeah, I got a 802.11a wireless card in it too...)