Always get good speakers/headphones first. Better speakers will give a much more significant difference in sound quality than adding in a sound card. Once you have good speakers then ask yourself "Am I satisfied with the sound that I have?" If yes, then you don't need a sound card. If no, then you might need a sound card. If you were to get a decent budget sound card, then I would recommend the ASUS Xonar DX. If price is not an issue, then ASUS Xonar Essence.
EDIT: For your situation, I think you do not need a sound card. Honestly, people usually get a sound card for music.