Getting well off topic here. 5.1 is 5 normal range channels and 1 low frequency channel. 7.1 is 7 channels with 1 low frequency channel. A stereo headset is 2 channels (left and right) any soundcard that says 7.1 has a maximum of seven channels and one low frequency channel but will do 5.1, 4.0 (Quad), 2.1, 2.0 dependant on whats connected.
You need to look at what you are connecting. A standard 3.5mm jack headset is usually 2.0 (Stereo) which means theres not real much point in you purchasing a sound card. You may be better off purchasing a USB based headset which will have its own audio processing meaning a soundcard wont be of any use to you.
Which headset have you got?