What would, in your opinion, be the slowest set of hardware (one
somewhat fast central server computer, and four somewhat slower PCs to
be used as terminals) you would be able to entice a teenager into using
for school purposes?  I was thinking in terms of cash-strapped African
school districts purchasing the new HP systems, and then selling them to
the more well-off school districts once local American charities started
sending them the kind of 5 used PC systems we're talking about here.

I have an LTSP term server set up for my siblings at my parents house.
The hardware on it is a Duron 750 with 512 megs of RAM in it and a few
old gateway P133 boxes for the terminals.  It ran with at least three
terminals just fine.  I'm pretty sure it could have gone higher.  Look
at ltsp.org for more on how to do this and what kind of hardware is
required.
This project has so much potential for the school computer lab/public
library terminal scenario.  It's much easier to support than a Windows
setup since you have one point of maintenance and patching.  
Have a good day.
Feel free to email me off list if you want more information.
Rob