Is there an 5.1 external sound card that I can buy for my iMac G5? I have the Logitech Z-5500 speaker system hooked up to my windows pc, but I'd rather have it for my iMac. I did hook up an optical cable, which work alright, the sound just wasn't as loud as the 3 cables that are hooked up to my win pc, (the 3 cables are needed for getting 6 channel direct output). The 3 cables are: 1 line out (front left/right), 2 line out (rear left/right), and 3 line out (center).