Personally I would wait and find out how the sound is in CotN before I would deside if I need a new soundcard or not.
16bit sound works on all types of soundcards made for Windows so no need to worry about that, the thing that might not be good enough might be the quality of the sound itself and if there are unwanted sounds generated by the onboard sound. All that depends on the quality of the onboard soundchip and the person listening.
A good soundcard cost about the same as what you have just paid for a new graphicscard.
Pleasant words are a honeycomb, sweet to the soul and healing to the body. Pr. 16:24