If the op-amp is in unity-gain then I don't see why it would be any worse than the voltage outputs of the chipset. No clear cut answers to your questions unfortunately. You'll never know for sure, you may get less noise or no noise with a good onboard audio solution or soundcard, but you'll never know til you try it. As it's a deal breaker of sorts to you, you need to make a decision on whether you want to take this out of the equation and save from the concern of it all for good and I can think of one way to do that; get an external DAC that is not susceptible to PC noise.

Can't help you any more than that, sorry.

-Raja