The sound card does not "encode or decode" the digital data. All of that work is done by the software running on the main processor. The sound card functions only as the A/D and D/A converters.
Some will tell you that the more expensive sound cards have less computer noise. My experience however is that the receiver background noise FAR overrides any internal sound card generated noise. Sure, if you disconnect the receiver and look at the waterfall you might notice a difference in background noise between sound cards, but connect the receiver and the receiver noise takes over.
The bottom line is that a standard sound card has plenty of bandwidth to handle the normal digial modes. You will probably need a better sound card if you are trying to run a DSP receiver.