The Astro 40 headset is pretty good so getting a sound card with it is a decent deal. For the most part any more you don't really need a separate sound card any more like the old days. Some people find the need for a sound card who want a little better quality or doing audio work, then it is justified but for gaming the sound chip on the MOBO is good.
But you could skip the sound card and just get an Astro 40 for less. The Mix Amp on the Astro are nice so get one with the mix amp.
Happy Listening, the Prisoner.