Well, the speaker u are looking at is quite good, but I recommend that you build your own sound system. I had a friend that built his own sound system (using just sb 5.1) and it sound wonderful (before he broke his soundcard somehow). so in my opinion:
1. Yes, you will get a better sound output by customizing your speaker.
2. There is a jack that will allow you to convert a speaker wire into a 3.5 jack.
But be reminded to spread your speaker around your room (Preferably hang them on the wall). And by doing this you will have to adjust your speaker volume manually (which is a bit annoying)but if you have a gaming keyboard that would be great since you can control the volume from the keyboard (especially when u're in a middle of a game).
Don't forget to protect your cables, especially the ones going on the floor (you dont want to stumble upon them and accidentally unplug the jack)
It is more expensive to build your own sound system u know...