Watching videos on a computer with an 800Mhz G3 processor is like going on Youtube in Windows 3.1. It won't work very well.
I would also recommend doing yourself a favor and looking for a better laptop (a powerbook G4 at least; those things are getting old enough to find in computer garbage). Ibooks are horrible. I repaired those things at work for over 5 years. Nothing but problems! They're poorly designed and poorly built.