My wife wasn't really too excited about the money I was spending on the sound for our home theatre. Once I got the 6.1 hooked up and we sat down to watch a movie, she loved it. She thinks it was worth it.
She hasn't been as impressed with the 1080p tv, which I just don't get. The picture is just beautiful. I guess she just doesn't see the difference in the definition.
I for one would really encourage the rear speakers. Also, that rear center speaker really fills in the room nice. I use my system for tv, movies, and gaming, not for music. I have a Yamaha RX-V1200 receiver and Infinity Entra for speakers.
As far as future proofing, maybe make sure your receiver has decoding for the HD sound formats, Dolby TrueHD and DTS-HD. You will not be dead in the water if you don't have them, but you may be thankful later if you do. Also, the more HDMI and component inputs the better. I don't know how many different pieces of equipment you have, but I ran out of inputs a couple years ago.
Btw, congrats on the new house.
