I've long assumed a good quality usb sound card would deliver better sound than an internal PC sound card, even of the audiophile variety. Why? Because of the inherent noisiness of PC's.
See then this quote from Tracer Technologies, who market both kinds of cards:
Many folks choose USB or Firewire soundcards for their desktop PCs because they believe that old adage that processing audio outside the machine is better. This simply isn't true for professional sound cards. In fact, given the choice, we'd always choose an internal sound card over an external for a desktop machine. It is simply more convenient to work with and given the quality of the shielding on the sound cards we sell, there is no advantage from a noise standpoint. The converters in our M-Audio sound cards probably better than in any of your other audio gear. You simple won't introduce noise on recording with these cards.
What do you think of this? Isn't there also the issue of the quality of the DAC in the sound card? Even if true, wouldn't the best sound come from the card with the best DAC, either internal or external?
What if one is using only the digital stream (say coax digital out) from an internal sound card: do noise issues of the PC have any effect?