No need for a computer card at all in most situations that I can think of, assuming this is a Windows or Mac with USB ver. 1.1 or newer connectors.
You should be able to see the USB device in your computer's audio settings after you connect it. Typically, you would set the volume to maximum levels on the computer, and then use the amp/dac to control how loudly you want to listen to your music or other audio. (games, videos, and other software)
For some sensitive headphones or in-ear buds, you may want to use the computer volume to adjust the sound levels to a comfortable level, but idealy you would want to keep it maxed.