I do own these speakers from last 2 years and they are wonderfull. I will suggest you first listen to these speakers for 1 -2 weeks ( Don't forget to connect in digital mode whenever possible ) and then decide if you need additional sound card.
It is always recommended to install sound card on ur mother board,even if it has an on board one.
A decent card from sound blaster(7.1 eax enabled)will cost u like 7000 at max,which will provide u the required digital outs.
So go ahead and get ur sound card and enjoy your games and movies in surround sound.
If u r using Logitech z 5500 you are better with these forums cause these are very good multimedia speakers. a lot of fight happened/s/ing from along time, so ignore that and read the innumerable threads to utilise ur system for full.