I am familiar with the audio pc's you asked about, and Onhwy has it completely right. Except for the advantage of being extremely quiet, everything else in the computer will be overkill for you except for the soundcard (you need a great sound card).
Audio pc's have their motherboard, processor, ram, hard drives, power supply, and even video card, optimized for multitrack recording and for the playing back of more than a hundred tracks at once while processing dozens and dozens of effects and recording overdubs. Certain motherboards for example do this better than others. Also pc builders know what components mate well with others. In the recording audio forums, there are known do's and dont's about matching components for a glitch free machine that can handle the above mentioned tasks. But if your pc is just playing back 2-track stereo audio, you will be wasting money on components whose potential you will never even begin to tap. Also you will be paying someone to match components that in your case don't need to be matched. ANY motherboard/videocard combination will play back 2 track audio without a problem, but if you're in a pro recording studio, the wrong video card can cause problems (I know, I know, I didn't believe that a video card could make a difference either at first - but it has to do with how the card taxes the system.)
You can get yourself a Dell and simply upgrade the soundcard/DAC.