Before you start going upgrade crazy, you might want to consider that your computer's sound problem may just be a speaker problem.
Computer speakers start sounding bad after to much use and a new pair could be all your computer needs. Below are the definitions for computer speaker terms that you should know, even if you are just doing a sound card upgrade.
Four-channel sound - This has two front speakers and two back speakers.
Six-channel sound (5.1 audio) - Six-channel sound uses front and back speaker pairs, an additional midrange speaker for the front or back, plus a subwoofer.
Eight-channel sound (7.1 audio) - Front and back speaker pairs, an additional midrange speaker for the front or back, a subwoofer, and a pair of speakers for the sides.