Why buy a sound card when you could buy a DAC (Digital Audio Converter)
http://www.newegg.com/Product/Product.aspx?Item=N82E16829132067
The inside of a PC's case is a very unhappy home for any analog device... There is all sorts of electromagnetic interference bouncing around the interior of the case. You should get a DAC and set it on the opposite side of your desk. That way you would have a Crystal Clear amplified headphone jack at the ready too.
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By the way, if you're looking for the absolute best sound you can possibly get... You're better off with headphones in my opinion. If you're willing to get the DAC and drop a couple hundred on a pair.
The Bose QC 25 headphones are unbelievable! The active noise reduction is ridiculous! I could roll a cherry bomb into your computer room and you wouldn't hear it go off. $299
...And by the way... While you can do some pretty cool stuff with 5.1 and 7.1 audio, it's really overkill.
You have two ears... You need two speakers...
Don't believe me? Put on a pair of headphones, close your eyes, and play this video.
[video="http://www.youtube.com/watch?v=IUDTlvagjJA"][/video]