Simutrans (both Experimental and Standard) still runs with code designed for 32-bit use (even when compiled into a 64-bit executable), so cannot make use of more than 4Gb of RAM. I have never seen a game, even a huge one, run out of memory yet.
As far as the CPU is concerned, I run an i7950 from 2010 overclocked to a little over 4Ghz, and that runs large maps fine. I have recently been working on improving the multi-threading support, so having more cores would be a useful thing.
Simutrans-Experimental is definitely more demanding on CPU power (and memory bandwidth) than memory quantity, however.
As to the exact minimum specifications, I have not done the sort of testing necessary to establish that: it depends very much on the size and complexity of the game world.