Getting a sound card will improve the sound definitely. I have the Asus Xonar ST and it has improved the complete sound for my setup a lot. Although it would make it very essential for you to match your amp and speaker combo with the sound card.
In my case my setup with Topping Amp + Xonar ST + Cambridge Audio dint match well (I found it a little sibilant) however, Yamaha A-S500 + Xonar ST + Wharfedale 9.1 matched very well.