Several cases can occur :

Good analog playback on good analog soundcard
Bad analog playback on good analog soundcard
Good analog playback on bad analog soundcard
Bad analog playback on bad analog soundcard
Good analog playback on good digital soundcard
Bad analog playback on good digital soundcard
Good analog playback on bad digital soundcard
Bad analog playback on bad digital soundcard
Digital playback on good analog soundcard
Digital playback on bad analog soundcard
Digital playback on good digital soundcard
Digital playback on bad digital soundcard
Skippy digital playback (bad on whatever soundcard).

Seriously, if analog is better than digital (and in theory, it's kind of wrong, since both can be perfect), don't listen to CDs, since CDs are digital !

In practice, the data on the CD can pass through some devices that will decrease sound quality :

1 The CD ROM drive DAC
2 The Soundcard DAC
3 The soundcard ampli

Using digital playback, or analog playback with a digital cable, you can bypass the device 1, therefore having a better sound, and go through 2 and 3 only.
However, using an analog sound card (such as the SB 64, for example), and analog playback, you bypass device number 2, and go directly from 1 to 3.
If you've got a digital soundcard, as it is nearly always the case nowadays, the device 2 can't be avoided since it's between the CD input and the line output on the soundcard. Therefore don't hesitate and go for digital playback (2-3), unless it skips (clicks in the sound). In the latter case, use analog plaback with a digital SPDIF cable, if your drive and soundcard support it.