if your an audiophile than a sound card would help since those are some great quality speakers...if not than its not a necessity. wattage should be fine since those speakers plug into a socket and sound card takes the load off your CPU but audio doesn't really stress your CPU that much...
Sound cards come with drivers that will allow you to modify the sound *equalizers* but for music and movies, there is no reason they should produce better sound quality then onboard sound.
However onboard sound usually won't process 3d sound in games. Sound in 3d games in affected in real time. Positioning, enviroment can alter the sound so some processing is needed for the highest quality.
Wattage doesn't tell you everything about the sound quality. For a bit more I'd get the z5500. It's a bit more powerfull overall. It also has onboard sound decoders and 3 inputs. But best of all it uses standard speaker wire from the sub woofer to the satellite speakers, a god send when mounting the speakers.