Bayne, nice mounting on those side surrounds. The 5.1/7.1 question(if you go with back surround you might as well use two, for a stereo effect with DPLIIx or Logic 7)depends more on how much space you have behind your sitting position for a back surround effect to develop, rather than simply room size. According to your numbers you'd have about 4 1/2', which should be enough to make it worthwhile. Separate them enough so that the usual listening positions get both a left and right rear effect.

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Enjoy the music, not the equipment.