Me too. Too much RAM 256 is okay but 512 is perfect for carPC use especially you only use a few apps at the same time.
You don't need 2 HD. Just buy one big one and partition it into 2: one big for media, and one small for OS and apps. Keep in mine that the PSU is limitted on it output. 150W is plenty of power for the VIA, but there are other devices as well.
You don't really need a USB hub unless you are running more devices than the number of USB port the mobo provides.
If you really want performance, go with AMD mobile CPU base.