If you wanted HBO HD you would need to get the HBO package, it gives you like 6 channels, one is HD. You can just try it out for a month, if you don't like it, cancel it. I get the ultimate package, which is my internet, digital cable with 4 premium channels. Each premium channel package offers a HD channel. It does seem that the premium HD channels are much better than the networks. They probably have more actual HD to broadcast since they mostly cater to movies. I have TIVO, so with 30 or so movie channels I always have something to watch. I already had Adelphia put me on the list for their DVR service so I can record, pause and playback HD content. I'll put the TIVO in the bedroom and the HD DVR on my home theater setup.
Which HDTV did you go with? I've had my Sony LCD 60we610 for 3 mths now and couldn't be happier with it.
I also find that standard channels look better using the composite video output, I have the composite on one input and switch to that when not watching HD.