chanster

A sound cards main task is to convert digital data from media source (such as CD, MP3, FLAC, APE, etc etc) into analog signal to be later converted into sound by a speaker.

while onboard and add-on soundcard has a similar task, those soundcards have different ways and tools to convert those data into signal.

and while onboard depends on CPU and RAM to process those, an add-on soundcard has its own processor and ram to do that.

if we're talking about greatness of those CPU and RAM, of course technically a core2duo or core2quad is greater, but please do keep in mind that those processors needs to process other tasks than just audio....
in gaming a CPU and RAM on PC has to manage perform tasks from the game software to produce complicated and stressful calculations and graphics.
while on the other hand, using a dedicated soundcard will release the burden of those tasks from your CPU and RAM and not to mention those add-on soundcards has above normal priority set for doing audio processing.
so by having add-on soundcards, at least there will be no more choppy and screechy or any other distracting/interfering sound

Now we move on to sound quality..
ever wonder what makes audiophile CD player is so damn expensive(such as Denon CD Player)?
basically a CD player is a DAC (or soundcard itself) because its task is to read from digital data(CD) and convert it to analog signal through RCA cables (Red and White plug cables)
those expensive CD Player has top notch components such as audio grade capacitors, Operational Amplifiers, and DAC Chips, so there for they produce far superior sound quality compared to other CD/DVD player you can find on the market(toshiba/or any other china brand)

You could test it yourself using the same speaker and you will notice it immediately.

now a soundcard is just the same as those audiophile CD Player minus the CD reader.
it processes those audio task dedicatedly, and using components made solely for that purpose.

Also a main disadvantage of onboard audio is the noise/electromagnetic interference of the mobo. A sound card minimizes the noise resulting in clearer audio.Some of the high end sound cards have a EMI shield to reduce the interference even more.....

this is not a snake oil, if you can demo/hear it for yourself, you should pinpoint the exact difference between generic DAC such as onboard soundcard which depends heavily on PC CPU/RAM with a great dedicated DAC soundcard.

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:::Creative Sound Blaster Timeline:::X-Ram:::The Difference Between CA20K1(X-Fi APU) And Everything Else!

92% of ppl have moved on to rap. If you are part of the 8% who still listen to real music, copy and paste this

CrackClocker

Good read!

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Jerrari

I love having add-on soundcard.
I use the Crytek Sandbox Editor a lot for mass physics stuff. anyway, I noticed that after I replaced my on board soundcard with an Creative Audigy, I could hear many more sounds (collisions between oblects).
Not to mention no more static!

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Crysis Physics Phun!