Thanks for replying!
The two I-beams are roughly 11", so if I flipped the room, it would seem really cave like at the back. My flooring will steal about 3", and the ceiling another inch or so. The other beam is going to be where a studwall goes to divide off the bedroom, so I really wouldn't gain much space, especially after putting in the false wall for my screen.
I do have two kids, and we do a lot of our movie watching after they go to bed. My biggest concern isn't noise entering the room, it's low frequencies rumbling throughout the house. My understanding from reading here is that unless you can do a lot of soundproofing, it's not worth the $$ and effort. I won't have room for isolating the walls very well because of the staircase location, though I could probably decouple the ceiling. I do plan on putting bass traps in the corners behind the screenwall, and anything else that seems like it'll give me the most bang for the buck.
As for seating, I'd like to be able to have four at a minimum, plus maybe some bean bags for kids when there are parties/sleepovers. I'll have to look for a love-seat style arrangement. I'd love to have some type of reclining arrangement, but it'll obviously have to pass the WAF.
I'm having the HVAC guy and electrician come out next week to give me some ideas and estimates. Luckily I don't have any existing ducting that'll be in the way of this room, but I do need some relocated where the bedroom will be, and obviously want to make sure that all the rooms have enough heat/cooling. I'll probably need to have the electrician put in a sub panel, and if so, I may put a surge protector in that panel.