the best place to start is the more-helpful-than-most faq's:
overall, it looks like you are on the right track, though powering the computer can be different-- mp3car sells dc-dc adapters for different motherboards-- and i believe that there are enough different solutions to power almost any setup you would want.
your dell link isn't going through so you might have to be more specific about what your looking for there.
for the via board--i have heard that via processors are slower than intel's similar offerings, but via purists say that they handle most tasks fine.
i have used a dual core atom board(the 945 in the store), and it was plenty fast running xp sp3, and some of my audio processing(very cpu intensive)-it was also a power sipper-i ran it for 3 hours once with the engine off with almost no effect on battery voltage(i think my yellow top dropped .5 volts..)- i ended up swapping it out when i needed to add more audio control, but the board couldn't handle it, and have been using it as a home computer--and it has taken everything that a home computer can do without a problem.
the primary reason for using a dc-dc adapter instead of a inverter is that it removes the extra steps-- for a inverter setup, you are going dc-ac-dc, and a dc-dc takes out the ac conversion, helping to eliminate some of the problems that sometimes happen in inverter setups..
for most, the on board audio is fine, though, if you have any special requirements for audio(active crossovers, computer controlled audio), then a external, or addon sound card can help.