Currently my HT setup is 5.1 and I am happy. Its there a benefit by adding the two back surrounds the only problem I see is that my couch is against the back wall and my surrounds are on the sides walls aim in an angle towards the couch and a bit to the center of the room. If I add the two back they will be place above the couch on the sides pointing downward to the center of the room.
I can get this one:
RTi4 for $119.00 a pair
RTi6 for $179.00 a pair
On sale at Frys also many other ones on sale.
RTi12 and centers