That mixer is made for 2 channel audio input. (left and right)
A 5.1 card would be a waste. I use a SB Live! Value for good audio quality, but come on, you will be listining to MP3s, not DVDs with 6 channels, no? If you did wish to use the mixer, you would use a 1/8th inch jack to RCA jack converter cable, (Sold anywhere, radio shack, Best buy....) to the main in, then take the sub, front, and rear, and amp it.