If you need an extra decoder to play back movies in surround, then the sound card you're buying is junk.
Heck, I know you need extra cables to connect a creative card for 5.1 for system sounds, I'd just pick up a Dolby Digital Live card like the Turtle Beach Montego DDL.
Anyway, you shouldn't need any extra hardware except for cables. But it took my friend something like 2 hours to get his Audigy2 ZX to work with his speakers.
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