the atom board will work fine for most carpc tasks such as music, and gps.
the m4 is way above what that board will need power wise,(a m1 will probably do fine) i had started out with a similar setup(atom board w/ dsatx) and it worked fine, but was nice when i had to upgrade to a pentium dual core setup for my audio processing, and didn't need a new power supply.
the choice is yours.
most car dashes will not have the room for a board of that size, and if they do, wiring, heat/air circulation, and serviceability can become a concern. i would recommend finding another location in the vehicle to mount the pc-- so that it gets more air, and is easier to service(and you will have to service it eventually).
also, i know the faq's are a little vague, so post up your questions if the faq's still don't make sense, and i, or others will see what we can do.
for the radio, i know that there are different radio options available, but i don't use any so i really don't know how good/bad they are. i know there is the griffin radio shark, directed/visteon hd radio hack, boomzbox hd, and hqct available..