that's an easy challenge to me. I used an iBook G4 12in 1.33GHz for 2 months last year in school with no issues until the end when it really slowed down (I was unaware of
@eyoungren's tweaks at the time) basically causing me to purchase my Dell E6410 that I love now. Now that ive gotten more macs and optimized them, I could easily do that "challenge" for more than a week. Good luck with it anyway!
Anyway, as for software, I keep it pretty basic. iWork and iLife 09, Optimized TenFourFox, Adium for IM, stock mail for eMail, stock iCal for calendar, and Time Machine to back up to my server. As for youtube, I have figured out that if you let the video buffer all the way and then play it, its pretty smooth. It still stutters a bit on my 12in PowerBook mostly thanks to the GPU (the G4 in it is 1.5GHz so that's sufficient enough), but it does work better than playing it immediately when you get to YouTube.