If you have Comcast high speed internet, you should be receiving an average of 6 mbs. Go to speedtest.net and check. I don't know what DTV requires for their VOD, but I never have any major buffering with my SA and I have Comcast. The one time I did, I found someone had clipped the exterior of the High Speed line outside and water got into the cable. It drove me down to around 1.5mbs. Check and see if that could have happened. Also make sure your router has at least Wep key enabled. If you live in a apartment or your homes are close together, you could have someone tapping off of you. You may also have others in your home downloading files while you are watching if you have a wireless set up.
As far as HD. TBN is the only one that claims to have an HD channel. The problem is most smaller ministries cannot afford HD cameras, so any Chrisitan HD channel will be basically a hybrid. The format of a preaching show tends not to lend itself to HD and SD should be sufficient.