Hi Chris (& all),
What I've read from Stereophile en heard in a specialized store here in Netherlands, the way to go is to use a DAC with an asynchronus USB port. Meaning, the DAC take over the timing control from your PC/laptop. This reduces jitter and digital noise.
These DAC's are expensive at the moment; it's a new technology. (Ayre e.g. 2700 Euro's in NL, cheaper in US, I guess). So, better wait. A good USB DAC for the time being is from Musical Fidelity (according to Stereophile and I know MF from their amps). Around $250.
Good luck,
Gerrit