Digital Decoders and Sound Cards

Interested in purchasing a new sound card and audio system....

I've noticed the Logiteh Z-5500 come equipped with a dolby digital decoder, however, would I see benifit from ordering a sound card with dolby and dts decoders built in?... or are the speakers fine by themselves. Do these sound cards do all the work thus making the speakers decoder unnessary?

http://www.sondigo.com/inferno

you're not going to decode things twice... so if you get those speaker I wouldn't suggest spending the money on a sound card that features Dolby Digital live