5.1 Surround sound from iMac.

Hi, I am wanting to change from a desktop PC (with Soundblaster card and Altec lansing 6 speaker surround sound (audio out from PC are FrontL-R, RearL-R, Centre/Subwoofer) to an iMac. Presumably I would use the firewire output (?) into some sort of adapter to create the audio signals descrbed earlier to feed into the Altec Lansing system.

I have seen old postings on the internet referring to a Griffin Firewave device but this no longer weems to be available.

Can anyone suggest how I might be able to do this. To buy a whole new home cinema system seems an expensive solution as the PC is only used in the home study.