Soundcards give you added functionality and connectivity, not extra quality. The inside of a PC is also very 'noisy' electrically, even scrolling a website can cause a whine on some setups. Having a soundcard can isolate things somewhat, or entirely with an external one.
If you don't get hiss, interference, etc when there's no sound playing then you don't have a problem though.
You might also consider a soundcard if you use headphones directly. A line-out is different to a headphone output which is amplified, and high impedance headphones (which are increasingly rare) require more amplification than the standard headphones outs on onboard audio or sound cards.
The function of either is to convert a digital signal to analogue which is a very simple thing to do, and the DACs in everything have been as good as each other for a long, long time. The actual DAC chip is inexpensive and very simple, costing pennies to a few £; lots of companies make them and each of these might list dozens of models with different applications - your ipod doesn't need a DAC that does 192/24 - but the same end result.
There is more to it than just the chip, you need to implement that into the full circuit with other components. However, look up DAC schematics and if you know anything about electronics you'll see how simple this really is. Although this latter part can be an area of contention with people into audio and knowledgeable about electronics as it's where you could create electrically measurable differences. Although that is very different to a change in sound quality or humanly perceivable differences.
But that's actually going a bit off track since both onboard sound and soundcards are both made by sensible manufacturers, not weird companies trying to claim they've used some sort of esoteric or exotic circuit design for their £4000 USB DAC.
Rather, they are both done in a competent and standard way and you really won't get a difference in SQ unless there is a fault or interference with one or the other. I personally say there's no difference between DACs,
and more.
Spend your money on transducers (speakers/headphones), there are very real variations with these. I used to change sources all the time and spent a fortune because of what people said, without hearing any changes really. Now I just spend a fortune on headphones alone... It's still foolish (you can be satisfied if you stick with something), but at least there is a difference to be heard.
Btw the people who say they hear a difference aren't wrong. It's just that it's normally placebo, think something will be better, or ought to be, and it is. Works the other way round though less effectively.
