Concerning sound cards I've read two major opinions that are seemingly rarely questioned even though they appear to be quite contradictory:
1. If you aren't a professional, you won't hear the difference between onboard and a sound card.
2. Using a sound card (especially with head phones) feels like stepping from 2d to 3d with huge improvements.
I used to have a soundcard, a creative soundblaster XtremeGamer, it broke after 1 year like everything else I ever bought from creative, but while it lasted it was awesome and when I first managed to get it working it really did blow me away.
I wouldn't describe myself as an audiophile and while my sound system isn't exactly PC speaker level it certainly isn't professional either, so what's the deal with that? Is it just the software? Psychological? Or is sound quality just not appreciated by the general public?
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The sound from onboard sound is lacking in punch and overall definition. Once you spend around $50 the difference between onboard and a sound card is clear... providing you are what I consider "nice stereo system speakers" and for me that means NOT PC speakers or cheap <$50 headphones. I have a 5.1 receiver attached to 4 speakers running in 4 channel mode from my Xonar DX. I love it, and the difference from onboard on a p67 chipset.
As for headphones, I think the the way sound gets rendered with the audio chips on sound cards produces a more accurate spatial soundstage, I just don't get that with onboard audio. I have decent ears, no golden record producer hearing, with fairly cheap sony headphones, the difference is not night and day, but it is certainly significant.
Sound cards do one thing important for me: they let me output a dolby digital signal from my games using a optical cable. If you think the onboard coax or optical jacks can do the same thing, well, you better make sure the motherboard says its a Dolby digital live jack or you will only get 2 channels.