The conventional wisdom (as I have noted elsewhere) is that PCs are so electrically noisy that they degrade sound quality. When Stereophile reviewed a PC sound card they found no evidence of this. The way I look at it is if they can't hear it with their audiophile ears and better equipment than I can't either.
Sound quality varies among motherboards. Try for a gaming board as they tend to be more expensive but better SQ.
Yes, it can be very expensive to install dedicated circuits for audio. Especially on newer homes built on a concrete slab. If you have a crawlspace under your house and the outlet is on the first floor then it might not be too bad. If you decide to do this ask the electrician to install a 3-wire circuit using metal clad cable. That will give you metal shielding and 2-120V circuits instead of one. It only requires one more conductor in the cable and one more circuit breaker but will double your power and reduce the voltage drop. This is especially nice if it's along way from the outlet to the electrical panel.
I personally think that the so-called audio grade receptacles are ridiculous. If you are concerned about this I would suggest a good Hubbell receptacle. They are bulletproof.
How to tell where interference is coming from is to move your audio and power cable around and see what makes the sound go away. Worked for me anyway.
