I don't know what your source material format is, but a very inexpensive solution that I recently stumbled upon is the Edirol UA-1EX. It is a 24/96 DAC, with optical digital, analog, and headphone output, and USB input. Very small form factor.
I tested it this week using HDTracks free download sampler record. The files are FLAC format. I added the FLAC codec to Windows Media Player (
http://www.xiph.org/dshow/), and played the files from my Dell Latitude 620, through the Edirol UA-1EX, into AKG 701's, and the sound was surprisingly excellent and could get very loud.
This is certainly a lower budget, portable solution. The Edirol comes with an ASIO driver. Worth listening to for the price (around $75). The more expensive gear will have more low level detail and somewhat cleaner sound, but at 24/96 the files sound very very good.
Bill C