There is talk of a digital low cost interface for PC audio to do HIGH quality audio via USB WHILE still using a dedicated DSP on the PC (e.g. a Creative SoundBlaster 64 or nForce2 audio, etc).
There are ways to do passthrough today for this stream, but you have to ensure latency issues are addressed otherwise audio will be a few hundred miliseconds off sync.
Your biggest enemy with USB speakers or headphones *IS* going to be CPU utilization as the OS handles the stream and this takes sometime away from anything else (e.g. more noticeable in some games)
Also if you have a digital output on your audio chip/card, try these

(expensive) but nice...
Sony MDR-DS8000 or the Sony MDR-DS5100
(Sorry no Sony link as their website sucks for linking:
http://www.crutchfield.com/S-2pUSnps...p?I=158MDRDS51
http://www.800stereo.com/prods/sony-mdr-ds8000.html
[This message has been edited by NickP 1K (edited 11-05-2003).]