I used to run it on 512 MB RAM pc 133, and it ran ok.. (with an old ATI 1050 series card); got a new machine with Vista Home Premium and 1G RAM and the sim took off. 38-40 fps in port, on the bridge, 70 at sea... went to 2 gigs of ram and even time compression has NO problems. Get 2G if you can, but these men are steering you right!
1G will make you happy.