The USB connection doesn't have to be associated with the sound card in order to give you the full sound quality that the headset is capable of providing.
Your sound card creates the sound in its DSP (Digital Signal Processing) section. The output of the DSP can be directed to several output locations. It can go to the card's own DAC (Digital to Analog Converter) stage, to provide analog output on the audio out sockets, or straight to a digital out connection eg S/PDIF. Or the output can be recorded straight into a file on the hard drive, or sent elsewhere via another form of I/O, like Firewire, Bluetooth or USB.
None of these alternative means of handling the signal have to be part of the sound card in order to work perfectly well. They just need sufficient bandwidth to carry the data rate for the sound, and USB2 has plenty of bandwidth for this purpose.
Edited by Platypus, 27 August 2008 - 10:00 AM.