Hi There !!
Well im a newbie as well to the MP3 car stuff, but have worked with computers for the last 12 years.
1. Size ?
2. What do you like to do with the carputer ? Only MP3 or , DVD, Divx and all other related computer stuff.
5. Future proof
My self it going for a Micro ATA of Flex Micro ATA board for a 478 P4 at 1600 Mhz , not the fastest CPU but it gives me the option to upgrade later.
Look at :