I am amazed at how much interest this thread has got in such a short period of time.
I would like to introduce myself, I am the owner and designer for HIPPOhifi. We recently launched a USB DAC "The Bloat", that essentially achieves what you are after except for the option of multiple outputs. I was referred to this thread by a very recent owner, Felix509.
The reason I mention this is not soo much as to try to sell you an USB DAC but to help you on your journey into the DAC world. Whether that is to create a custom design or review another design I would be more than happy to help. I have read a number of the posts and want to reply to nearly all but that just isn't feasible. So feel free to fire some questions. I will start replying to a few now.
PS. I also have a CarPC, my turn to return the favor after nearly downloading all the Road Runner skins.