SIRIUS Stuff should start appearing in a month or two. I just posted the specs and protocals for the Gen 2.5 tuners last night. I am hoping to have SIRIUS and XM control software written for my car sometime...
Nav software? No clue, haven't gotten that far.
Sound Quality? IMHO, I think using an external USB Audigy 2 would be keen! It gets away from the noise inside the pc. However, if you plan on spinning up MP3's it might not matter.
Mini ITX boards? I stayed away due to cost and performance. I want a Pentium 4 processor and wanted to stay really clear of the VIA processors (yuck!). I went MicroATX ASUS board (ASUS rocks!) and a desktop case. A bit bigger than I would have liked, but everything is standard computer parts (no slim cd rom drives, no special fans or cooling). I paired it up with an internal 802.11g Extreme 108mbit PCI card. But in the end, if you go MiniITX plan on spending double for the cost of the computer.