Okey Dokey
So I'm assuming the room you'll be using is the "Recreation Room" seen at the top of the pic?
I can see how it's a bit tricky. The placement of the doors (what looks like a closet and then the door to the "Unfinished Area") and the placement of the fireplace means that arranging your seating becomes the real trick.
I don't yet know all of the considerations you may have, but I'll throw out a few ideas and then you can give some feedback and we can go from there
1) Use the 50" plasma as you've planned. Put it on the wall that is to the left of the fireplace (that would be the left side wall when looking at the pic). You could then put a sectional couch that would have a short section on the bottom wall (to the left of the door to the unfinished area) and a longer section that sits into the room and would directly face the telelvision.
This setup would give you a large seating area, it wouldn't block any doors and you would essentially still have half the room behind the couch to use for other purposes.
On the downside, the TV and fireplace would compete for visual attention. This is an interior designer no-no, but if you don't care too much about that kind of thing, then it should work well.
2) If your fireplace allows it, you could go with the ever-so-popular, but less than ideal from a home theater standpoint "over the fireplace" arrangement for the TV.
A couch and one or two chairs could allow for a genuine diagonal arrangement, with the couch placed diagonally in the room to directly face the telelvision above the fireplace.
The other seating option would be a sectional along the left side and bottom walls, but this would give you a much longer distance from eyes to screen and no seat that directly faces the TV.
3) You could go whole hog - completely ignore the fireplace and actually position the room so that the fireplace is behind your seating and out of sight. The way you might do this would be to use a front projection setup instead of a plasma. With the prices on 1080p front projectors these days, the cost wouldn't be much, if any, higher!
You could place your front projection screen on the right side wall (directly below the staircase entrance in the pic).
I would then recommend either a couch with chairs, or better yet, a curved set or home theater recliners! Position them so that they are just to the left of the two doorways (to give clearance for the doors).
This would give you a seating distance of about 13 feet or so from eyes to screen. A 100"- 110" screen would be ideal for this seating distance.
One sticking point here would be where exactly to position your speakers. The screen itself would be about 7.5' wide, so you would only have a couple of feet to either side of it. You wouldn't really want the left speaker to be right up against the wall - which would be the only way to avoid blocking the staircase entrance. You could move the speakers quite far forward though and actually have the left speaker infront of the staircase entrance.
Another possible setup would be to have the screen and speakers both infront of the staircase entrance (in other words, about 3 feet to the left of the right side wall). The way you would do this would be with a retractable screen mounted to the ceiling. Think how cool it could be to press a button on your remote and have your projection screen decend from the ceiling for movie time
Anyways, those are just some thoughts...
Let us know how those sound and we'll work out the details more clearly after that
