That set up seems prime for a 6.1 experience.
The 6.1 and 7.1 set ups are very similar. First, there's not that much separation between the rear channels in a 7.1 system. The information is all derived from another speaker (be it the surrounds or a matrixed stereo source).
Looking at the back wall, there might be a problem wiring a left rear surround because of the door. Therefore, go with the 6.1 installation, using a QS8 or 4. The wide dispersion should make it seem like the whole back wall is covered in sound. This will given you that convincing enveloped feel you're seeking.
I do tout the benefits of 6 and 7.1 over 5.1. I think that the directionality of sound in the rears becomes less distinct and the panning is better. I think that most people's rear set-ups dont let them experience the benefits because their couch is butted right up against the wall. Your situation is different, and you'd likely notice and enjoy the upgrade.
In short, go with the 6.1 and a QS of some sort. You'll be happy. Consider, however, getting a receiver (unless you already have one) that will give you 7.1 eventually, just in case you get upgradeitis or discover a way to place a second rear surround. Just make sure that it has Dolby Pro-Logic IIx -- notice the (X) in that, very important.
Anyway - have fun. It looks like you've got a great room for HT, baby!
