It depends on the HD channel! Different providers use different bitrates, and most of the time they're variable.
For example, here in the UK. HD channels from terrestrial have an average bitrate of around 4Mbps, but they drop as low as ~1Mbps and can go as high as ~15Mbps. That would mean you'd need an upload speed of 15Mbps to avoid any dropouts.
My upload speed is 20Mbps and I usually have no problems with this!
