To add to what MiniRock said, I believe (someone please correct me if I'm wrong) that the latest flavors of Dolby and DTS synthesize a rear channel on discs that don't have a discrete channel.
There has been some discussion in the past couple of months as to whether listeners have found 6.1 or 7.1 advantageous, with mixed reviews. A Search in the forums will turn up some valuable opinions from those that have walked the path before you!


::::::: No disrespect to Axiom, but my favorite woofer is my yellow lab :::::::