iMac Sound?

I have a logitech 5.1 surround sound system. Does anyone know how I can get it running considering I cannot put a surround sound card into my iMac. I know almost nothing about different sound jacks. Does the iMac have another slot apart from the microphone jack?

Does anyone have any info on what cable I would need to hook it all up.

I have something very similar to this..

http://www.logitech.com/index.cfm/speakers_audio/home_pc_speakers/devices/234&cl=us,en

As long as it has an optical input you are fine. It also says it can do it with just the 2 channel input.