@Drew - I am using the Audigy2NX. The sound system update (Pioneer DEH-5800 headunit, Rockford Fosgate 4004, and CDT/Elemental Designs speakers/subs) definitely made things louder and much, much better sound quality.
The Parrot works with the Treo ok (no picture ID though), but I don't believe I'll be able to control it with the pc. PhoneControl didn't work well with the Treo at all.
The latest project is finishing the fab needed to cover the components in the trunk, and diagnosing the headunit's crappy radio reception using the stock antenna wire. The reception problem may have to do with the headunit's proximity to the rest of the equipment in the trunk. I did get the girder/UIRT control of the headunit to work perfectly.