I posted this somewhere else but it still works better then mactubes for me so I will re post it here.
To get low CPU usage while watching YouTube you need to use safari and have the develop menu enabled. You enable this by going to safari preferences then the advanced tab and then check Show Develop menu in menu bar. Then open the develop menu on the top right of the screen and click user agent, then Safari iOS x.x.x - iPad. Next open a video page and If you have a little slower ppc machine make sure the HQ button is unselected below the video. Now when you watch YouTube, videos will play in QuickTime on the YouTube page and you will still be able to see comments and have a nice browsing experience. Videos should play with a very good frame rate and there should be little video stuttering. This works very good on my 1.33ghz iBook G4.