It is easy to make good DACs these days. Basically any DAC, barring a messed up implementation, is likely to sound sonically transparent to any other in a normal system. When you look at the other limiting factors (amp, noise in the room, speaker response, room reflections, etc) you find that their noise and distortion are just way below audibility. Ya, maybe if you have a really nice setup with a quiet treated room, good amps, and have it set for reference (105dB peak) levels you start to need something better than normal, but that isn't very common. Even then you usually don't have to go that high up the chain to get something where again the DAC is way better than other components.
Now that said, there can be a reason to get a soundcard given certain uses. For example you don't always want to go to an external unit, maybe you use headphones. In that case, having a good headphone amp matters and onboard sound is often remiss in that respect (then again, so are some soundcards). Also even if you do use an external setup, you might wish to have the soundcard do processing of some kind. Not so useful these days, but some games like to have hardware accelerated OpenAL.
Regardless, not a big deal in most cases. Certainly not the first thing to spend money on. If you have $50 speakers, don't go and buy a $100 soundcard. If you have a $5000 setup, ok maybe a soundcard could be useful, but only in certain circumstances.
As a side note, the noise in a PC isn't a big issue. Properly grounding/shielding the card deals with it. A simple example is the professional LynxTWO, which is all internal yet has top notch specs, even by today's standards. http://audio.rightmark.org/tes...