1) Should I get all electrical outlets in that room switched to 20 amps, or just one?
Just to expand a bit on that- it depends how much gear you have. With my basement installation, I'll have one dedicated 20A circuit with about 4 duplex outlets. So, all my HTcomponents, DLP projector, Video game console will be plugged into these receptacles.
Do I need it? for now, probably not; but I did this more for the future if I decide (and have the funds) to buy a more powerful system. Also, because this is new construction, its easy (no retrofit or demolition) and doesn't cost me any more than a 15A circuit.
4)Should I get a few quotes before I proceed ?
I received several quotes for my entire basement wiring. And I can tell you, its all over the place. Try to find an certified electrician who doesn't penny pinch- an example would be setting a price for each pot light installation. The electrician I hired gave me a flat fee (materials & labor)- when I asked him to install one dedicated 20AMP circuit, after the fact, he said no problem. He was honest cuz he knows the labor isn't any different and the increase in material cost is insignificant, if any. Also, I revised the lighting plan to include 6 more pot lights and again, he still went by his initial quote.
Bottom line- get a few quotes and if you can get a referral somehow, even better (that's how I found my electrician).
Looks like your doing a bit of retrofit, so i'm not sure how big a job that would be. You would have to run 12 gauge wire and change the outlets and breaker to your 100A (or 200A) service. So, yes, I would think that some areas of the walls will have removed to install new wiring. It really depends on where your room is in relation to the main power panel.
My question is: if you're room is a good size that can "handle" a beefy system, then it may be worthwhile. But if your room is fairly small, then I don't see the point.