On music storage:
For better or worse, most of my digital music is on iTunes. I currently have 4 PCs as well as 3 ipads sharing the account. For transferring music to the car, I just plan on having itunes installed and let it grab whatever latest tunes were purchased over wifi while parked in the driveway. My plan is to keep a copy of the entire library on the HDD in the car. I would hate trying to decide beforehand what I might want to listen to, then running back/forth with a thumbdrive. I also I have a lot of old LP's recorded as uncompressed .wav files, so I'll need lots of storage for those.

On the DSP:
I'm planning a bi-amped (or maybe even tri-amped) system + subwoofer. I haven't yet looked at a "PC only" solution but I'd need 4-stereo channels, each with it's own x-over and eq. I have experience with the 2x4 miniDSP and am really happy with the performance/cost. The 2x8 nanoDigi looks similarly easy to deal with.

I still have lots of planning and research to go before I buy everything, so I'm definately open to suggestions.