I was playing 720p video successfully on the iMac G5 before it was chopped. It was a 1.8 GHz model.

You will want to use VLC, which is faster than QuickTime due to the ability to disable the block filter. Go into the preferences and disable the H.264 loop filter, then play your 720p QuickTime video which will run flawlessly (other than a slight loss of perceived quality due to the lack of block filter)
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