I am not competing (yet?), but I do want good SQ from my carputer system. I should have it installed within the next 2 weeks, so I'll have more info then...
I chose to use the m-audio sound card (7.1 isa card). It has really good specs and good reviews from users. It comes with some basic sound processing sw, but I may also be looking for more if I am not happy with the sound.
Another important factor is how you store your music. Mp3 can be good or bad, depending on how it's converted. I may use un-compressed wav files, which will sound great, but take up a TON of hard drive space.