Is there a noticeable improvement of sound cards vs the onboard audio?

What motherboard do you have? Most of the "good" modern ones come with 8 channel audio built in. If you are an "audiophile" you probably have a separate "home" audio system already that you could input your CPU audio through if desired (music, movies, etc.) If you are just using it for games most would find that the "on-board" sound of today's motherboards is sufficient.
Having said that, the card you picked out won't bust the budget and if you feel that you need it then I'd say get it.  

It depends on how old you are.

And I personally would take the on-board Realtek ALC889. You have a separate 2-channel stream off the front panel header for your headphones in addition to the base 7.1

Sound is difficult to quantify "quality" you can't really measure it (without some serious fidelity testing equipment anyway). If you don't have any notable problems, just stick with onboard audio, because these days it's pretty good.

I personally use a Striker 7.1 board because it was available cheap, and I had issues with the onboard microphone input. Rather than replace my headset with a USB, I just slapped a decent sound card in to resolve the issue.

Oh sorry, I forgot to tell you that. The motherboard is: Gigabyte GA-P67A-D3-B3. Yeah, I'm talking about listening to music and watching music not just playing games.

So my onboard audio is good enough for high quality music(like .flac and 320 bitrate mp3s)? Or at least good enough for my headphones?

Realtek ALC889

Educate thyself and calm yer harsh-ed mellow  

It would be good enough for me(and probably 99% of people) but some people are really picky. It all depends if you classify yourself as an audiophile or not.

Believe it or not more will depend on the source of those .flac/320 bitrate mp3's than on the equipment playing them. If you want to correct some faulty/noisy recordings you could use available software (most good players are already equipped to do that). Some tweaking on your part might be all you need. The audio on-board today is miles ahead of where it was years ago. The best I could offer you is try the on-board and if you don't like it, and you know its not your speakers or the quality of the recording, try the sound card.

If you get a good sound card you can put a better system on it rather than just sticking in the green plug. I have a sound blaster titanium that is 5 yrs old with a xfi front hook up. I have a cheap surround sound sysyem hooked up to it. Im not an audio tech guru but after 3 or 4 beers when you want to crank up the Led Zep or whatever you hit on You-Tube you will be very happy. If you listen to rap or country please stay with the on-board!

Ok, thanks for your help guys, looks like I should just stay with the onboard. From what I've seen some good sound cards cost a lot too, about 100$, I think my carppy card (asus gts450) costed about that much.

also

lol'd

Depends on the speakers/headset. I can say you won't see any improvement using .mp3's due to their compression, but anyone with a FLAC library and decent speakers should invest a little in a soundcard.