laptops would be the worst type of computers to use for a constant music source, they get hot, fan gets really noisy, and the life of the computer will be shortened. Heat is the worst thing for a laptop. why do you think they make those cool pads with fans in them, so they can cool your laptop. I know, i have a macbook and have friends that have macbook pro's. if you look at a true server type of machine, you will find multiple fans or in the case of the mini, a design that gets the heat out.
there was a review of the airport express a few years ago that a stereophile guy wrote on the airport express using his mark levinson dac and he raved about it. a good dac or a great dac will improve the sound whether it is from a toslink or coax cable more so than a $100 usb dac will provide.