If your current soundcard does not have digital audio outputs and you are connected with analog audio then one that does should give you an improvement in sound. That assumes that the Denon has digital audio inputs.
Just adding to what americanaudio said...
I would suggest getting one just for the digital audio outs but only if you don't have them already. If you do need one just get a decent one since your receiver will do most of the work anyway it just needs the signal.