get to a point late in the game where you are getting invadded a lot.
open the task manager.
click through several dozen invasions.
go back to the task manager.
you will see that memory usage steadily increases while going through the invasions. eventually, the game runs out of memory and planetary invasion no longer works (transports go up to the planet but no invasion happens). at this point, several other dialogs don't work, including starbase construction.
I have 2 GB of real RAM and a 4GB swap file. should be plenty...