1. Being backed up against the wall needs to be avoided, however, it does happen a lot. Some people put up massive broadband absorption there, and a few might put the couch on sliders so that they can get off the back wall for critical listening. The bass will collect at this boundary, and in turn will mask your other frequencies, dialogue included.
2. Depending on who you ask, someone might say the allocation is very dependent on the size of the room. The bigger the space, the more of your budget might have to go towards a sub. Heck, if you asked me, the bigger the space, the bigger the speakers might have to be. There are a lot of decent systems you can put together with your budget, I'd take a look at Ascend and PSB for instance, but really there are many more, just keep reading.
3. I am guessing you are seeing what is called Frame Interpolation. That "soap opera" look? I hate that crap. You pretty much only see that with LCD TVs, never a plasma so far as I know. FI simply guesses what the in-between frames are, by using algorithms. Back in the day, with fast video, the processor couldn't keep up, and so a running kid's shoes would look all funky, or a football in the air would turn into three footballs etc.
Well, I know certain TVs like Samsung's AMP (auto motion plus?) have different settings. Me being paranoid, I'd like to know if it's at least completely* defeatable.
How much would max setting apply? I don't know, but I can tell you the potential. If I take a bluray or DVD (24p), and have a 120hz TV, that means I can potentially FAKE OUT 96 frames for every second.