The Denon receiver has a coaxial digital input that you can use with the PC. You will need a mono mini plug to single RCA plug on the cable. You will need to check the instructions for the soundcard but usually the front stereo out can be changed to the 5.1 digital out in the creative software for the card. You will also need to assign the digital input to a input button on the receiver.
the photo is a bit small for me to read the text but is that a spdif input i see on the receiver? if so why wouldnt you just use the spdif output on your soundcard and connect the two. you can set spdif as the default output under sound devices.
possibly a better option is to run both your audio and video to the receiver on a hdmi cable and then run the hdmi out cable to your screen from the receiver. if your video card has dvi ports and supports sound out over dvi (most newer and decent brands do) you can use a dvi to hdmi cable. this is how i have mine setup and it works great. the receiver will change to be your default audio device.
i see no need to resort to coaxial or rca. by doing so you will be sacrificing sound quality anyways.