5.1 surround can be done through an amp (A5100T 5.1 surround amplifier)
Im going with XM not FM but here is a tune http://www.hauppauge.com/html/usb_data.htm
For an audio knob im getting one a Matrix Orbital MX214. Its an LCD screen that Shows all sorts of information. Its progamable and has a winamp plugin to it.
There might be other products out there to control volume but i havent found one i liked better than this. I plan on putting this on into my steering wheel. If i cant get the cell phone to integrate into it first. Be nice to have it work for both though?????
And i would use and external sound card with a line out to a signal processor like the Audiocontrol Overdrive then to an EQ like the audiocontrol 24xS. This would provide an awsome 13volt signal and some ultra clean sound set perfectly for your particular vehicle