There are two things to consider here, sound card & speakers. If you have cheap Pc speakers then a sound card alone will not make much diference. If you want good sound you need good speakers. At the end of the day it's the speakers you listen to. An expensive sound card with cheap speakers will not produce good sound. If you have good quality powered speakers then a better card might make the diference.
Hope this helps/makes sense. Cheers Lixdexik.