Building aHT

I am planing to do an addition this spring and really could use some advise on the basics as far as requirements to make a reasonable ht. Size, will a 12' X 15' room be adequate or if slightly larger what would be the ideal dimensions. The equiptment will be rt2000 mains cs400 center, rt10 surrounds (small towers) and I have a 35i for a rear if I can use it in this space. I have a Denon 3801 and will be getting a widescreen directs HDTV.

Any reccomendations or suggestions on sites concerning soundproofing would be most welcome. I am starting from scratch here and on limited time so would really appreciate any help available.

Thanks & regards
EJ