I have a dilemma if i should purchase a usb soundcard for my current HTPC setup. My future setup will look like this.

Mac Mini (intel core duo)
Logitech Z-5450 (via optical)
Sharp D43 series 32inch LCD

would a usb soundcard benefit the decoding that is already on the Z5450? or its a waste of money? please give suggestions. what soundcard to buy?