My room size is about exactly the same size, with the monitor in the middle of one of the longer walls. My couch is against the opposite wall, with QS8's mounted on that wall on either side of a window above the couch. This is my first HT setup, and I'm quite pleased with 5.1. Not sure if a 6.1 or 7.1 setup would work in my room, but I don't feel a need for it.

I have M60's, VP100, and an SVS PBI-1SD sub. Sound is fantastic. I got a Yamaha RX-V1400 receiver online for about $600. Here's my first post after I got my system set up, with a link to some photos further down the thread.