Welcome to the world of CarPC
I too am about to embark on my car PC build for my Civic Type R. I am in Europe so my Civic will be different from yours I guess.
I agree with your goals although I am willing to sacrifice some sound quality, for me it needs to be good not perfect. Some additional goals for me are Navigation, quick startup and reversing aids.
I have already started some sourcing of my components, I have initially tried to keep costs down my using some existing equipment lower end components, if I am successful with my install I will upgrade later.
I have a few comments on the hardware that might help, of course these are only my views from my experience.
I am currently using a Intel D945GCLF2 with 330 Atom and 2GB Ram. I have to say that I am quite please with this board, I know it is not the fastest chipset around, but so far I have found it adequate for a car install. One benefit is that it does not get very hot. Once running the Front End the performance for Audio and Nav is fine and the videos I have run ok. The other nice thing is that the BIOS (although not very fast) is quite silent (very few POST messages) and it is also possible to implement your own BIOS boot screen, I have made this the same as my XP Boot and Login screen so the boot up looks quite consistent. I would like to try a core 2 in the future but for now I am happy and would recommend an Intel board.
M2-ATX-HV 160W Intelligent DC-DC PSU, this is what I have and it is excellent.
I would have at least a 7200RPM HDD, I have noticed big differences when trying out a 7200, 5400 and 4200. My start up times are currently around 26 secs (including the M2-ATX 5 sec delay, POST and Windows start). Return from Hibernation is about 20 secs and return from standby 3 secs. I will be investigating the use of SSD in the future.
I will also be experimenting with just 1GB RAM soon to see if that helps Hibernation times.
Front Ends. In my view there are only 2 worth considering. Centrafuse (CF) or Road Runner (RR). There are lots of threads on these so I wont rewrite what has already been said. I am currently using CF, I find a bit more polished and stable than RR although RR can be customised a lot more.
I am currently using a Voom2 Case, It is an ok a case, things get a bit tight in there and I have had to rerun the cables to make it tidier and replace the fans as they were too noisy. The case fits perfectly in my glove box and is very tough. My only concern would be putting in a Core 2 Duo, I think it would get very hot, anyway this is something I will try in the future.
I hope this helps you a little, as I said it is only my opinion on the topic. Keep a look out for my forthcoming posts on my build in the next month.