well you only get those premium channel HD's (hbo, showtime, starz...) if you subscribe to those channels. To add to that you will get TNT HD, ESPN HD.
We may be missing some. the best thing to do is go to
www.comcast.com and look at the channel lineup.
To directly answer your question, no you cannot pick and choose what channels you get, you can buy the HD package which will give you the locals in HD, but also a few extras (that is probably what you are looking for)