mini-ITX is the way to go IMHO.
Not only less power needed, but generally less heat needed as well.
You have to consider being able to power a larger board with the avaliable PSUs.
Why are you bent on the X-FI card? There are other cards out there that are just as good if not better...even in USB.
Regarding the USB PCI card...use a powered hub....simple.
Your comment about cases has be slightly worried...are you just planning on sitting a tower in your trunk?
There are cases out there (Opus) that have 2 PCI slots anyways and include the 2 PCI slot riser card (1 PCI slot -> 2 PCI slots).