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That's the analogue output. In that case, kinda depends on what you are feeding it in to (what speakers or headphones). The output on the Realtek HD audio isn't horrible, but it is built with price in mind and as such the quality isn't everything it could be. A good dedicated card can improve that. However if you have something like cheap $40 computer speakers, well then probably you don't notice any real difference. The money would be better spent on better speakers instead. If you have something like ATH-A700 headphones, then yes you'll hear a difference (in the case of those, less hiss and more bass will be the most noticeable effects).
Considering you can pick up a Xonar DG for about $20, which should handily outclass the onboard sound, it is probably worth it - but it depends on what you are listening to, and through.
The difference depends not in specific codec model or brand but in the actual motherboard implementation. If codec was sufficiently electrically (noise) isolated from other parts - then you don't need sound card, but still often its not the case - case in point my AsRock P67 B3 Extreme4 Mobo - Connecting my powered monitors created terrible ground look and was unusable until I filtered my AC outlets and switched to pci sound card (details in sig)
However if you do not hear noises in your speakers then hard drive reads or mouse movies or buzz/humm from speakers then nothing is playing - then most likely sound card (especially cheaper model) would not make your sound better.
If you use digital out (optical/coaxial) to a receiver, amp, etc. then a sound card is a waste of money. For analog and/or headphones, a sound card is far superior to onboard.