I installed Ubuntu in Virtualbox and set the virtual machine later to 64 MB of
RAM base memory. This was enough to start with the Fluxbox session and then
start Nautilus from the command line. I had already slammed a Firefox launcher
on the desktop and I could load Firefox easily.
Now I am writing under Ubuntu installed in Virtualbox, with base memory set to
64 MB.
I also tried Ubuntu with 32 MB of base memory but it did not even load (of
course).
I am going to experiment further with lightweight window managers.