I've gotten really good results with my Lenovo laptop using a NuForce Icon HD USB DAC/headphone amp. It's the same DAC they use in their $1500 CD player and in my system it rocks. It can currently be bought new for around $300 on Amazon or NuForce.com. Nuforce also makes a slightly more expensive version that adds S/PDIF inputs (RCA and Toslink) if you need more than just a single USB input.
new system/older equipment
I have my music on itunes on a macbook 2.16 ghz laptop. Playing it through an old, cheap ($50) eridol ua-1x dac into an Adcom GFA 523-II power amp and an Adcom GPT-400 amp/tuner combo, playing through Alison 7 speakers. Since I went to the macbook from a good cd player the sound is thin and unsatisfactory. I just added a trial download of Amarra Hi-fi and that helps a bit. I presume that the next thing to do is to replace the old crumby dac with a new one (Dragonfly?). Can anyone make any other suggestions to improve the sound. Other than the Amarra Hi-fi, is there another reasonably priced music player that works with the itunes interface. I've burned most of my cds with Apple lossless, but I could easily reburn them with another codec if there is one that sounds better. I have lots of space left on my hard drive. Thanks -Howard