I have old pcs that I have since installed linux on and they've never run smoother. One's an IBM netvista and the others Compaq. (I have sworn not to use Dells) But my favorite "old" computer is the powermac g3. The monster is a powerhouse. It has served as a great backup for our main home computer (g4 mdd) and we never turned the thing off.
My favorite is "Pentium I", my first PC.
And now I'm using Pentium IV...don't laugh, my family is poor. And besides if you're still laughing then I can only say that for a programmer you don't need much resources.
Programmers don't need much, yes, unless they're trying to make some game like Crysis, that's where they're going to need a lot.
I'm curious, what Linux distro do you find to work best with older pc's?