You will get somewhat better SD quality using component vs s-video, but it won't be HD quality; component provides more bandwidth.
Another factor is how good the video scaler in your TV is. It will rescale the video input to match its native resolution, and a good scaler can give you pretty impressive results, considering the signal source. If your HD receiver has switchable output formats, try letting it do the scaling, and then compare that with the TV's scaler.
Using a calibration DVD is a good idea, because TVs are usually preset to look good at the store, not in home conditions. However, one thing that hasn't been pointed out, none of the current calibration DVDs are true HD (DVDs aren't HD). So, you can get a good calibration for SD and DVD playback, but the HD calibration will be a bit of a compromise. Or, you can pay a lot and have your set professionally calibrated.
Some TVs have separate settings for each input (my Sony does), so you'll have to do multiple calibrations if you use multiple inputs. Yes, this is all a pain, but it'll be worth it. Now if there was only some HD content that was worthwhile......