I'm dealing with the same issue myself. Here is a shortlist of ones I'm considering:
Silverstone EB-01 (super cheap at around $90, cool looking, apparently decent, but takes power from the USB connection)
Firestone Audio Fubar II (around $120, tiny, well reviewed)
EMU 0404 (Around $200, gets loads of good press on head-fi)
EMU 0202 (Around $130, has a different DAC chip to the 0404, and lots less features - but not so much information about performance relative to its more expensive brother)
and probably the most attractive option of the lot for me...
DIY Paradise's USB Monica board ($145, but requires some DIY work to hook the DAC up to power and put it in an enclosure - this is the only NOS DAC on the list, and is claimed to be the only USB NOS DAC that does not go through an SPDIF conversion).
Chad