Because you can do all of your processing on your amp, you should'nt require too high end of a car PC. Keep that in mind when shopping for CPUs.
Do you want to keep your system stereo or surround? (I would hope stereo as five of your six listed uses are stereo) What speaker set up are you planing? Active or passive crossovers? Sorry about all the questions, I like to see systems running the DC amps.
The Xonar is a nice card, but it may not be the appropriate tool if you are using stereo media. Either way however, onboard is acceptable for calls... if anything, I would be concerned with mic line noise if the PC is far away. You could give a USB mic a try.