<< Judging by the age of your users and the software you're going to be running, you don't need anything fancy, but what you do need is something reliable and simple.
I'd get the MSI K7N420 Pro motherboard. It comes with GeF2MX graphics, LAN, sound, NIC, and modem. It's an all in one solution that takes no more than one driver install. It's simple, stable, and cheap ($128 at Newegg).
For the CPU, I'd go with the 1GHz Morgan Duron processor. It's fast and cheap ($40 at Newegg). >>
Ditto, installs a a breeze, driver upgrades are a breeze, great quality graphics and audio
~$130 mobo
~$40 CPU
~$65 256MB DDR
~$10 floppy
~$30 CD-rom
~$60 Antec case and 300W PS
~$15 cheapo mouse and keyboard
~$10 cheapo speakers
~$100 17in monitor
= $460
At this point, your better off buying a budget dell or gateway WITH the OS and some software (Food for thought)