Towards the top of the 2K limit I would check out the Simaudio Moon 300D. At a lower price I would try its little brother, the Moon 100D, or a used Apogee Mini-DAC. An Apogee Duet is an interesting option if you have a Mac, especially if you want to record as well.
However I have to agree with Larryken and Daverz: if better fidelity is what you want, you need to rip at the highest resolution you have room for. Any of the outboard DACs mentioned above will make you music sound *better*--even squished mp3s--but if you want *more* music you need higher resolution.
One last thing. Specialty playback software can make your music much clearer-sounding. On a Mac, these are the free (for now) AyreWave from sbooth.org/AyreWave/, PureMusic ($129) or Amarra (price varies depending on version).
Another last thing: don't forget a good USB cable. My stock cables sound lifeless compared to the silver one I bought from Audiogoner Acreyes, and there are many other good ones available too.