Hey shieldwolf.
I have those same speakers for my PC. The green cord is for the 2 front speakers and the purple is for the 2 rear. You could buy a y adapter and plug them both into your existing soundcard. However, the front and rear speakers will play the same thing. I don't know what you are using the speakers for but that may not be a problem. Instead of using the adapter, you could probably spend around $30 on a pci soundcard that would have both the green and purple out.
Good Luck