How about some actual answers for your questions?
1. You probably can't. Getting HD OTA is a good first step. You can also use your HDTV's QAM tuner to pick up most of the same OTA stations "in the clear" IF you keep your cable for internet only. If you ditch cable entirely, you only option is OTA. OTA will provide the basics: ABC, NBC, FOX, CBS, in most cases, PBS, and a few others. So don't worry about watching your favorite network tv shows, sports and news. You'll be able to get them OTA. CABLE tv is where you will feel it. This mean all those other shows you may like: Comedy Cental, Sci-Fi (BSG), TBS, TNT, History, Discovery, etc...you won't have real time access to. Boxee and Itunes are your remaining options, detailed in #5.
2. Streaming: You keep all of your TV content on a constantly running computer, running itunes. This computer "streams" all the content you want to watch to your ATV. Yes, for this to work, you are forever "tetherer" to an online, running PC (or mac) at all times. Syncing is just like an ipod. Your computer will transfer content to the ATV itself. Since all the content is on the ATV's harddrive, no computer needs to be running in order to watch this content (after it's been transfered). The problem with syncing, is the relatively small HD size on the ATV. You will find yourself streaming eventually, because if this, most likely.
3. In most cases bittorrent would involved pirating. In which case, it's illegal.
4. Itunes: almost every show, both cable and network, can be purchased from itunes, directly from, and to your ATV. In other words, you can browse, buy and watch all without interruption, from your couch. For some of those cable shows you just have to have, or if there is value to you in owning the shows to watch over again (or let others watch), its a good deal. For $1000, you can get anywhere from 30-50 full tv show seasons. Thats a lot of TV, for less than what cable would cost you all year.
5. Boxee is a 3rd party hack, developed as media center software (think windows media center), that can be installed on the ATV. Boxee is not approved or supported by Apple in anyway. It is not illegal to put Boxee on your ATV, and you will not void any warranties by doing so. Boxee allows you to stream online content, straight to you PC. One of the services Boxee used to allow you stream, was Hulu. Go to Hulu.com to get a feel for what it is. Basically, you can watch most Fox and NBC shows on Hulu for free. This service has now been blocked from ATV. There is still CBS, Comedy Central, WB, MTV and some others. With ABC coming soon.
Basic rule of thumb for us. First and foremost, try to watch what you can OTA. If that fails, try Boxee. If that still fails, purchase via Itunes. I promise, with this method, you will spend FAR less per year on TV than you did with cable.
Oh, and if you live of ESPN, Comcast sports, NFL Ticket (or other sports packages), you are pretty much entirely out of luck as of this time. This is not the route to go if you are a heavy tv sports junky...
Hope this helps...