Ok, for funzies I ran Navigatrix on a machine with 256MB RAM and no hard drive installed.
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With a terminal running there is about 9MB free memory to work with. Open up OpenCPN and a terminal, for example, and it drops to a bit over 5MB. Yes it can be done. I wouldn't want to do it...it would drive me up the wall. But that's mostly a personal statement.
If you kill the GUI and run a straight console....
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you gain 20MB worth of free memory to conquer the world.
...it's not worth it. Yeah, you could start killing unused processes like bluetooth...because you wouldn't be running bluetooth on an antique anyway...to gain more free memory...but it's not worth it.
If you don't have 256MB off the blocks, grab your sextant, paper charts and use another distribution like Damn Small Linux.