I am not one of those who thinks there is a big benefit for 7.1, especially in the typical size room people have their HTs in. I've gone back to 5.1....:)
I've used both single monopoles and a center in a 6.1 setup, and I felt that using a center speaker worked better in my room. Another possibility would be to get another pair of 160s and use them for the centers. FYI: you don't need to get a new receiver, 6.1 and 7.1 are the same thing...