I have a P3 800 mhz, 512 mb and video card 128 mb.
What's the best OS to install in my CarPc?
I recommend XP Pro, but it boils down to what you are comfortable using and whether your software is compatible with that OS.
Have you looked in the FAQ yet?
How about the Wiki?
Under normal circumstances, a signature would go here.
Windows XP is recommended in most cases unless you are shooting for a lower memory footprint (Windows 2000) or higher stability (Windows Server 2003).