Does the receiver support gigabit, if the receiver does not supprot gigabit, I don't think you will gain much going to a gigabit switch, the receiver will still only handle up to 100mbps, after you factor in overhead, you'll get about 60-80 mbps. I tried hooking my office computer that were were 100mbps to the gigabit switch, and it did not do a thing. However, if you have a hub, and not a switch, upgrading to a switch should improve performance.
60mbps is more than enough to carry HD, so it sounds like the the problem if it is network related could be coming from other traffic on the network. I don't think that gigabit is necessary to stream one HD show from one box to another, consider putting your receivers on their own subnet, (and switch) so they don't share traffice with other devices in the home.