For what it's worth, I just managed to make the client run on a 256 MB heap. Therefore, if there are any of you trying to run this game on a low-memory computer and having performance problems, you may want to try this client:
http://plymouth.seatribe.se/java/salem-lomem.jnlp
It does some tradeoffs to support its smaller memory footprint, however, so it might lose in performance in other areas. Try it and see. :)