the sound blaster MP3+ USB with the RCA connectors has extremely high quality output
you may have a problem with the motherboard emitting high levels of EMI (electromagnetic interference) try to position the sound blaster USB sound card on the same plane as the motherboard, beside it (not located anywhere on top of the motherboard.
you may want to put the sound card in a metal casing instead of the original plastic thing (4 screws on the bottom).
you may have some problems with your computer's USB chipset.
try the car pc at home plugged via RCA on your stereo. if you hear noise in that setup, you have a much bigger problem...
another idea is to use some sort of capacitor across the carpc power supply
also you want to connect the carpc and the amplifier's power supply to the same source (that is the same fuse box), so as to avoid the potential ground issues