As the amps these days use HDMI switching (as said earlier), use HDMI for everything that has it. For Sky however, also use optical for the sound as they only transmit 2ch via HDMI for some reason. On a Blu-ray, if you use optical for the sound you won't get HD sound as optical can't carry that much information so you will be limited to DD/DTS.
Although there is plenty of chat about the differences (or lack of) from one HDMI cable to the next, its not as simple as black and white (or 1's and 0's ). If you have a recent TV that supports Audio Return Channel, then you will need a 1.4v cable to utilise that function to an audio system.
On the amps, I personally think that the Yamahas and Onkyos are the two current best mainstream brands. The Yamahas have the edge on surround DSPs and the Onkyo the edge on 2ch audio. With the RXV673 winning the What HiFi? award, the Onkyos have dropped in price quite a bit, so are amazing value. All IMO of course