Do you need an amp in addition to a DAC? If so, there is a Beresford TC-7510 Mk6/3 in the For Sale-Source forum for $100, which is a great deal for the unit. I have the same model, and with my roomate's HD555's, I found it a considerable upgrade to coming straight out of an X-Fi Xtreme Music soundcard.
The only downside I'd say is that the Beresford is not USB and requires a digital output from the computer (either digital coax or Toslink), which not all PCs have, but is a fairly inexpensive upgrade.
With dual inputs and outputs, the TC-7510 also gives you a lot of versatility if you want to use mutiple sources or speakers, in addition to headphones.