So you are looking for a better sub woofer? Or just better speakers all around? What is the sound that you think they are lacking?
For a better sub sound you can buy a used cheap stereo/mono receiver/amp(or even make an amp} and a used cheap home stereo speaker and can get some decent sub sounds, and just use your other speakers for front and rear. You can make your own. Parts Express has good values on buying woofers or just do it yourself speaker building.
For a less then 40 bucks for a soundcard Turtle Beach are good. You won't hear a great difference in sound. So unless you can spend more you might as well keep the card you have
on your computer. Which value card do you have? Their has been a few versions.