Well, for speakers like that, I wouldn't recommend anything more expensive then the ASUS Xonar DX. [I'd also recommend the D1 and HT Omega Striker, but lack of PCI slots make those no-gos]
And soundcards do make a huge difference when paired with a decent set of speakers, despite the mantra many repeat. To me, playing without a soundcard is like playing with AA turned of at 1024x768. You can, but the final product is ugly.
I used to have a soundcard in my old PC. Although it was only worth about 30$ it definitely made a huge difference, which is why I agree with your comparison Gamerk316.
Would you recommend a different set of speakers? The logitech z623 got great reviews on a lot of websites i checked out, but i do not know anything about speakers. Please tell me if there is a better price/performance set of speakers out there.