Alot of people on here (myself included) actually splice into the molex connectors from the PSU that power syour hardrive, cd-rom's etc. The reason behind this is that the PSU supplies a clean feed of +12v while your vehicle electrical system typically runs at around 14v mark. The added benefit of this method is that the screen will only ever be turned on when the computer is so you don't have to worry about relays etc...
You just need to make sure your PSU can supply enough juice to power yoru computer and screen (which my m2-atx) does easily).
*edit* If you do go down the relay path, then an SPDT is all you need setup as below
*edit2* Beaten