I got a Thinkpad 760 used for $50.
Came in very handy for 3rd and 4th year university. (Computer Science)
Its loaded with a Pentium-166, and 36 megs of ram
Upgraded it to a 2gig harddrive recently.
More than enough for taking notes, playing SE4 and keeping up with the forum
You will need something beefier to play SE5 on, though.