If you don't mind running some wires, I would suggest changing the orientation of the room so your display device is along the 12' wall. In fact, this weekend I finished converting my den which is roughly the same size as your room to a listening/movie room. Even though I had a window to contend with where I wanted to mount my plasma, I used the longer dimension for my room length, allowing me to install a complete 7.1 Axiom system.
If changing the orientation is not an option, I agree with the previous reply and would stick with a 5.1 system using stands for you surround speakers.
Good luck
