The screens that come with your car or your head unit or your GPS are video, not VGA screens and are much lower resolution than VGA.
This post has pictures of what a computer output looks like on a video screen. This uses an additional piece of hardware called a scan converter to clean up the picture as much as possible.
Take a look at the desktop below. Note how little space there is between the Start menu and the taskbar icons on the right. There is only enough room to display two running applications. Note also how few icons are on the desktop and the fuzziness around the text. The reason for this is that the display settings have been cranked way down to something on the order of 640 X 480 or less.
This is necessary because the video standard is nowhere near as crisp and clean as a VGA screen. That means two things - you display less and what gets displayed doesn't look clean. With exactly the right skin and colors, you can make it work, but just recognize that it is a compromise.
In addition, you can see that the desktop is cut off slightly at the top. It is theoretically possible to fix this but the video standard works in such a way that it is intended to cut off a bit all the way around because early TV sets had a bezel around the tube.
Note in this image how much screen real estate the Google page takes up. Almost the entire vertical and horizontal space. Note also that the text appears fuzzy. As a comparison, load Google on your VGA computer screen and shrink the window down until it is roughly the same appearance of this image. That's how much space you'll get on your car PC desktop if you use a video screen.
Bottom line - if you have a very specific type of installation such as just using Road Runner as a music player and perhaps video player, then a video screen might work for you. You can control the size and color of the text with a special or simplified skin that reduces the blurriness and deals with the color problems and handles the cutoff around the sides. Navigation is possible, but you want to be able to tweak the font sizes, colors, etc.
However, if you want a full up PC with web browsing, intend to use the desktop, and so forth, the limitations of the video screen will be revealed to those who check out your installation. It will make it look cheap because of the fuzziness of the text, the washed out colors, and the tiny desktop size. That may be okay with you and if you already have an OEM screen in your car it may be worth it. But if you are just trying to save money on your installation by going with the less expensive video screen, realize that it will be obvious that you took that path.