"In short when will most of the channels be available to me in high definition?"
In answer to this, probably not for quite a while. Considering cable companies have upwards of 300 channels, I would consider "most" to be the majority of them. Currently at best it's close to 10. I expect in the next year or so, we could see this number double, but I don't think we'll see it go beyond that all that quickly. There are no mandates for broadcasters to broadcast in HD, just mandates for them to be digital.
Either way - I personally still think it's worth it. Most of my viewing is done at night on the major networks. Only thing I miss occasionally is the WB. TNT and TBS would be nice too. And SCFI. That would cover my needs

Battle Star Galactica in HD would be quite nice!