Are you having troubles with streaming videos or with videos you've downloaded?
If streaming videos are causing the problem, there isn't much you can do.
Today's technology for streaming video maybe be outside the scope of what your computer can handle.
bajoro,
You don't specify the CPU speed and the RAM that your iBook has. Generally speaking downloaded QuickTime movies will run better than streamed videos.
My son downloads torrents of his favorite TV shows and watches them on his eMac 1.25 GH 1.2 GB RAM with no problem.
Richard