Before you spend a bunch of time and money on speakers, wires, and a clean installation do a test to make sure you want 7.1. I don't want to discourage you from upgrading to 7.1, I think it's great. Some people have reported they don't care for 7.1 or find it unneccessary.
Try taking some speakers you have laying around, don't worry about timbre matching, and set them up behind you on some chairs or stools. You can run the wire across the middle of the floor. Listen for a day or two to be sure the added sound is worth the investment for you. It was for me, but if you search for 7.1 you'll find others who'd disagree.
Jack