Dusty, hello again. Since you've ordered QS8s for side surrounds, the next step in my view should be the main speakers. Unless you have a compelling reason to use on-wall speakers I'd suggest that you consider the M22 instead of the W22, or the M60s if you decide to go with towers. You can temporarily do without a center speaker and have the mains form a "phantom" center. The receiver in the HTIB system can temporarily power the new speakers satisfactorily.
As to a new receiver, in the room sizes that you're speaking of don't worry about "a good bit more power"; it shouldn't be needed and any of the receivers with ratings in the 100 watt area should be fine. Even 200 watts would only mean that a brief split-second peak in the sound could be played 3dB louder, e.g., 108dB instead of 105db, a slight difference. For a receiver with more up-to-date features, including decoding the new audio formats(actually not essential if a player did it)consider units such as the Onkyo 706 and Denon 2809.
On the 5.1 or 7.1 speaker setup question, it isn't so much the size of the room which should be the primary determining factor, but how much space you have behind the listening position(at least 4' would be desirable)for a rear soundfield to form.
