Well, even though PC sound cards have gotten a LOT better, there still will be some amount of noise (for the audiophile). Using optical out should eliminate any noise generated by the PC. You could also look at a USB external solution, where all the audio is generated on a seperate box.
In terms of the network, a Linux box would be rather expensive (in terms of heat produced and electricity consumed). Do you actually need enhanced routing capabilities, or would a cheap gigabit consumer router work? Even if you lose 1-2% of performance, wouldn’t a consumer router be worth it?