Have you given much thought as to what sort of monitor you'd use?
Going from DC to AC back to DC is really going to hurt the efficiency. What sort of PSU have you planned on using? I'd be inclined to look at a DC to DC PSU. There are a few available here:
http://store.mp3car.com/SearchResults.asp?Cat=22
I've never used this product before, but it looks like it might suit your needs provided you could adapt the connectors (or that they fit as they are):
http://www.sarrio.com/sarrio/12voltlaptop.html
If you're using a battery to power the system, you'll really need a 'PSU' which regulates 12V (something like the picoPSU won't, it relies on the 12V being constant). Batteries aren't always good at maintaining constant voltage, which could be quite dangerous to a computer (although there are ways around that as well, it involves adding more circuitry). Make sure that the DC-DC PSU can deal with a reasonably wide range of input voltages.
As you've probably gathered, this will end up being quite expensive. Additionally, you can expect a typical 17" LCD monitor to draw around 35W, which will push up your power consumption quite a bit. A 90Wh battery (if you're given capacity in Ah, just multiply that by the output voltage to get Wh) might only power it for ~1.5 hours.
If you're not so fussed about a battery, something like a 120W picoPSU might be a good idea for you:
http://www.mini-box.com/picoPSU-120-power-kit?sc=8&category=13. It's small, very efficient, relatively cheap, and should provide ample power for the sort of system you have planned.
Hope this is some help to you.