To be sure (there seems to be problems with surround in some games, e.g. Mass Effect, if you don't have a dedicated sound card), and since my home theater system after all isn't exactly low-end, I purchased an Auzentech X-Fi Home Theater HD, which solves my problem. Thanks for the feedback! :)
I'm working on connecting my gaming rig to my home cinema equipment for a better gaming experience. There's 15m cabling involved, and it won't be a permanent setup (the cable will run straight through the living room when I'm gaming), so for expenses and convenience I don't want to use more...