In 5.1, you have front speakers x2, 1 pair surrounds, and center speaker, plus sub for 5.1 sound. With 6.1, you add a rear center speaker. With 7.1, it's like 5.1 but an additional pair of rear surrounds. So, you sound card might end up sending the rear center channel to one of the surrounds, seriously mucking it up. I'd say stick with 5.1, since it's the most you would need. 6.1 and 7.1 are only necessary if you're a total audiophile with an acoustically perfect room (using reflecting sounds and distances to get better sound emulation), and I doubt your room fits the bill. 5.1 is the most that a casual user needs for the forseeable future.