Re: I'm curious about the quality of the audio, myself...
I've had this problem with internal audio on PCs *and* Macs for as long as I can remember. It is very noticeable on good quality headphones and reasonable hi-fi gear. Realtek audio is bloody awful.
Interestingly I even had a noise issue on an M-Audio 2496 in a G5 Power Mac. There was a distinct "ticking" particularly on Line In.
The only way to solve this is peferably, galvanically isolated USB DAC / ADC using a separate linear power supply injected at the USB socket of the device.