My old msi z77 mpower has THX capability, as does my old Onkyo receiver, and the new Onkyo I looked at a week ago. The problem with pc's is they are very seldom used for Cinematic sound reproduction, so THX has very limited use there.
The Z906 are THX certified, which means they are compstible for use on a pc that runs THX surround, but it's not necessary or needed to get full use from them. They'll work just as well without it. You don't need a THX sound card, but if you did have one, you could use those speakers with it.