Check out the iFi line if you just need to use USB from a computer. No frills but very good sound using the Sabre line of chips.
In terms of relative quality to using a CD player, it depends a lot on the CD player you have and the computer you use (and the software). Check out computeraudiophile.com for a number of different excellent computer configurations for digital audio. I have truly found that eliminating moving and spinning parts (drives, fans, pumps) and going with a low wattage/heat system can make a significant difference in the sound. You also don't want that computer doing anything else than playing the music.
In terms of relative quality to using a CD player, it depends a lot on the CD player you have and the computer you use (and the software). Check out computeraudiophile.com for a number of different excellent computer configurations for digital audio. I have truly found that eliminating moving and spinning parts (drives, fans, pumps) and going with a low wattage/heat system can make a significant difference in the sound. You also don't want that computer doing anything else than playing the music.