Yes you are right, so long as it's connected correctly and that your Yamaha amp is capable of upscaling.
Quality of the picture is a personal and quality thing though.
I run Sky HD (720p) direct to my Plasma and the picture is superb, the sound is handled via a QED Optical lead for DD 5.1 which again is impressive. This way I do not have to have my amp on for the 10 o'clock news etc.
Hope this is of help.