Do you want a sound card that is for games or for music. They are extremely different.
If it is just for games, the sound improvement will add some effects, but you would need an x-fi. I got mine for gaming, but I have played less than 50hrs of games in the last 3 years so I am rather unimpressed with their quality for music playback.
If it is mainly for music, what type of music and what types of files? If you are listening to low bitrate mp3 files, then nothing will likely help, and improvements could just make it worse. If you are listening to flac, wav, or 320mp3 (to some extent), then you could go hog wild with sound improvements. (that is a very dangerous road to follow) However, your $50 budget will not net you much of anything. Most people who are after high quality musical reproduction will use an external usb dac -> amplifier->monitors for 16bit 44.1khz tacks and a soundcard with optical out->external dac -> amplifier->monitors for 24bit 96khz tacks. This type of setup will net you improvements, but will start around the 3-400 range. You could cut the amp and get an internal dac, but truthfully, your speakers are not designed for musical reproduction. The ESI Juli@ is supposed to be a good budget conscious internal dac that can do 24bit 192khz and retails for around 150, but I have no idea as to what it will do in a gaming environment.
If you just want gaming effects, I would hold off until you can get a mid range x-fi card which will run you around 100. If you want music, then I would recommend looking into headphones as you get much more for the money and an external dac, but it may take a few hundred to realize improvements depending on your tracks.
also read this thread and if you are feeling inclined, you could hang out in that forum for a while.
http://www.head-fi.org/forums/f46/true-adudiophile-card-343287/