If you are going to use a computer as the source an inexpensive DAC with USB input would do it for you. There are dozens of them between $150 and $500. A few I've used are the original Musical Fidelity V-DAC, Valab, Beresford, Aune and the Audioengine D1. I like the V-DAC the best among those but at $170 the Audioengine is really nice.
If your computer is a Mac you'll probably be happier with the optical output than with USB in an inexpensive DAC so having an optical input on the DAC would be a consideration there, one of the reasons for the recommendation of the Audioengine D1.
If you're using a streaming source like Pandora I would strongly recommend you try mog.com. I was a devoted Pandora One listener for a long time but the much bigger selection of music, the ability to better control what I'm listening to and the clearly better sound of the 320kbps stream from mog have made me a convert.
If your computer is a Mac you'll probably be happier with the optical output than with USB in an inexpensive DAC so having an optical input on the DAC would be a consideration there, one of the reasons for the recommendation of the Audioengine D1.
If you're using a streaming source like Pandora I would strongly recommend you try mog.com. I was a devoted Pandora One listener for a long time but the much bigger selection of music, the ability to better control what I'm listening to and the clearly better sound of the 320kbps stream from mog have made me a convert.