Some quick questions regarding speaker/sub woofer placement...

[Big Daddy]

It is ok to install your surround speakers on the back wall as long as they are not dipolar speakers. For dipolar speakers, it is best to install them to the side of your main seating area.

Try not to put your surrounds too close to the corners as they tend to emphasize lower frequencies and sound a bit unnatural. Also, don't point your surround seakers down. Point then straight toward the front of the room.

As far as a subwoofer is concerned, there is no easy solution. Each room is different. What works in my room is not necessarily the best option in your room. Anybody who tells otherwise does not fully understand the concept of standing waves and room modes. There are two threads with sticky on them: A Guide to Subwoofers (Part I) and A Guide to Subwoofers (Part II): Standing Waves & Room Modes.They may help you in placing your subwoofer or they make completely confuse you.