Ok, so there was an unplanned stress test last night at mineserver.be-server
I don't have an idea how many players were in at a time, but server started to kick out players at some point because of the userlimit (which was 50), then I changed that to 500.
Max ram usage I saw was around 200MB (before it segfaulted!). There is a known bug which causes segfaulting all the time when there are people in and teleporting =/
I have some pics here:
http://fador.be/mineserver/stresstest/btw, number of player files in world/players/ is 392 at the moment...I believe it was below hundred a week ago. (No Notch!)