HRT - MUSIC STREAMER II STD or NuForce UDAC-2
Anything else will be waste of your money
p.s: I dare you to compare above to $27 Behringer UCA-202 (thou it's only 16/48 it's better than CD quality)
@BSA: I think it would be great for this site to do some comparisons on inexpensive dacs; the Behringer might do very well in a test. One caveat though is that the headphone output has 50 ohm output impedance. That probably won't work well with your typical headphones that has 16 to 32 ohm impedance. Also I have some doubts about the nuforce udac-2. I've owned one and used it extensively and got rid of it before buying an audio-gd fun "b" dac for not much more (it runs around $200). The udac's volume pot is only so-so; the audio gd fun has high and low gain, variable and fixed out, 3 digital inputs, a heck of a lot better volume pot, and a markedly better sound stage. And audio-gd, unlike a great many chinese audio companies, has a very good US distributor, Pacific Valve. I think you can do a lot better than the nuforce udac-2 at around the same price.
One last point, if the Behringer does 16/48, as your post states, that may not be a good thing since redbook is 16/44.1. I think it's worthwhile to have bit perfect audio output in computer playback.
I agree about the HRT musicstreamer II as I've recommended it more than a few times. It is made in CA (everything else being equal I prefer to buy something made in the USA), it is asynchronous so the dac handles the audio timing (a computer does not do a great job on this), handles hi res files very well, and does a reasonable job of isolating the audio stream from the computer which tend to be fairly noisy environments.