The minimum requirement of Trisquel is probably due to LXDE. An Openbox-only
(or Fluxbox, Blackbox, IceWM) "environment" should need even less processing
power. The very last resort would be textonly mode, for that you really don't
need hundreds of Mhz.
I don't understand why network is a requirement, it should not be so. I have
an old PC laying around with Win98, it has 500 Mhz but even less RAM than
yours and it has no network card as well. I'll try it out soon and see if
there is an option to install without networking abilities.
If everything else fails, Debian should be easy to install on old PCs,
because you can choose to not install a desktop environment. From the text
only mode you can then install and test any kind of window manager you want.
As long you don't download from the non-free repositories of Debian, you'll
remain 100% free.
