Minimalistic Linux desktop
A major advantage of the The GNU Operating System is that you can piece together the programs you want and create "your own" OS with the building blocks you see fit. These are suggustions on how to create a Minimalistic Linux desktop which runs on old hardware.
 Window manager
Suggestions included evilwm, pekwm, XFCE and, oddly, KDE.
Note that use of an integrated environment, be it KDE or Gnome, can be more efficient since the apps share libraries. But resources are eaten when one tries to mix and match KDE and Gnome and wind up loading both sets of libraries.
Conkeror may be a nice choice, an offshoot of Firefox with keyboard-only navigation modelled after emacs. But it is still Firefox under the hood, and eats resources.
Opera may also be a good choice, it is fast and has tabs. But it is not free software.