You aren't alone at all friend! I've been searching the internet most of the evening tonight trying to find something, but this is the only post I can find related to this problem.
Not to be a jerk, but I've looked into the system requirements on Hulu and Youtube. A 1Ghz G4 iBook isn't too old to be playing videos online.
Youtube requirements:
http://www.google.com/support/youtube/bin/answer.py?hl=en&answer=78358
Hulu requirements:
http://www.hulu.com/support
My mother was having the same issues. What bugs me is that I've used the computer on Hulu and Youtube previously without any issue. I cannot thing of what changed to make this happen. So I maxed out the RAM on the iBook 1Ghz G4 (running 10.5.6) to 1.125GB (buying a 1GB stick off of newegg.com), downloading Safari 4 Beta, the most recent Firefox and updated to the newest version of Adobe Flash Player as well. Nothing worked... at most it is slightly smoother. I wish I could say I've had more luck with this, but I haven't. Hopefully someone else out there might know something. I'll keep checking back. Good luck.