So I planned to buy an external soundcard for my Logitech z906, but I can't see the difference between the Creative Soundblaster X-Fi and Creative Soundblaster X-Fi HD (except the price). And another question, do you guys recommend it for this setup?
Unless I'm missing something here, you're looking at the old models of external sound cards from Creative. With all due respect, I'd suggest looking into an internal sound card if you're working off a desktop. If a laptop, then you should look outside of Creative like the Xonar from Asus. Again, I'd like to work with internal sound cards(or the onboard) before dropping more money on a sound card.
Is it just for music or do you need surround sound? Because if you don't consider getting an audio interface rather than a "soundcard". For the same money you generally get better sound quality, and better connectivity.