Unless your CD player is very high end, a seperate DAC will usually produce better sound. Bryston makes a very well reviewed and regarded CD player, which is, in relative terms, a very reasonably priced player. But they also have a seperate DAC you can buy, that is a juiced up version of the CD player DAC, and IMHO makes the CD player sound better. I can tell you that with computer/media server based music it is a necessity if you want good sound to use an external DAC.
My Oppo disc player sounds OK on it's own, but through the DAC that I have, it sounds great, well on that rare instance I actually play a disc.