The DAC in the PS Audio will likely be of higher quality than most soundcards. Feeding the PS Audio a digital signal over USB should sound better than feeding it an analog signal from a soundcard.
If you have a soundcard with a digital output then the difference would be much smaller since you'd be using the DAC in the PS Audio regardless of which connection (USB/coax/toslink) you used. Toslink and coax sound a bit different, so maybe USB would too, but I'm not sure if better or worse.
One word of caution: I use a Macintosh with the highly regarded M-Audio 2496 soundcard (coax digital out) as a jukebox and it sounds markedly worse than my DVD player (also use the coax digital out) Bits are supposed to be bits, but to my ears the computer does not make as good a transport.
