MONITOR: I would recommend you use an external touchscreen monitor. the eee is a laptop, and one of the biggest advantages of a laptop as a carPC is the ability to use/configure/upgrade/etc it outside of the car. if you hack up the eee, you'll lose that ability. but of cost is your deciding factor, then adding a touchscreen to the eee itself would be the cheapest option.
OPERATING SYSTEM: for carPC use, windows XP is widely preferred as the OS of choice. but it depends on what you need your carPC to do...
if it will just be doing audio and video, then the default Xandros Linux will be fine. it even provides a nice "big-button" front end for you to use. if you want to run good GPS software, etc, then you'll have to switch to windows. (although they are improving, there are currently no good GPS solutions for Linux).
