Oh yeah, and if you add the Mo-Pingo Puc's it will improve the sound. :)
They do make a tube sound card out there, but I think I leave the tubes and their heat in the basement where they can warm the ambience of the concrete walls.
If you have a digitial noise problem, due to a cheesy power supply, that can make a difference, shielding of the PS could help.
The processor speed can cause glitches, and speed of your modem if you stream audio will be a factor, but frankly if your streaming audio, quality is not a word I associate with that sound. If your using it as a server or CD player it's different. More significant will be the amplifier section. I would try and pick up an outboard amp, and leave the high voltage outside the PC.