hi john - here's some general answers.
1. you want to be approximately 3-5 times the screen size away from the tv. so, for a 50" tv, you want to be about 150" back - approximately.
2. most gear's width is standard - right about 17". for depth, it depends. my 6-cd carosel changer is pretty deep - i think it's around 22". some receivers are pretty hefty and deep as well. i guess around 25" or so should be safe, but you'll have to take into consideration what you'll be putting in.
3. you should be viewing the tv dead-center from your viewing position - that is - sitting on your couch. in my case, i lucked out. on my couch, when sitting, i'm nearly dead-center to my mits 65".
since i suspect you may also have questions about speaker placement, i'll save you a post.
http://www.dolby.com/ht/Guide.HomeTh....html#chapter3