judging by what your want, here's your complete recommended setup:
athlon xp-mobile 2200+ (35watts processor)
biostar m7ncg mobo (micro-atx, nforce2 graphics chipset)
512mb ddr ram
whatever size your hard drive (7200 rpm)
xenarc 700ts (get a 700tsv if you want video-in for rear view camera)
opus 150 watts dc-dc
casing : whatever you want, you can try doing it yourself too
usb wifi (optional, i recommend d-link)
usb bluetooth (optional)
usb gps receiver
as far as your sound setup goes, here it is
creative's audigy2nx is a 7.1 channel sound card. you will just be using 4.1 channels. for output to the amps, there are 3 stereo out ports on it : front, rear, and center/sub.
get 2 amps : a 4-channel amp for your cabin speakers and a 2-channel amp for your sub.
get a stereo to rca adapter to connect the front to the first two channels of your amp, and the another stereo to rca adapter to the last two channels of the 4 channel amp. set the 4-channel amp to a high-pass filter to cut off the bass that the subwoofer should be responsible for.
another stereo to rca adapter connects from the audigy's center/sub output to the 2-channel amp to drive the subs. set the amplifier to a low-pass filter so only the bass comes out.
have i spoon fed you enough?