For people using 64 bit systems, you might want to try this utility found on this page. Can be a bit hard at times to get this page open, but when it does, look in the Utilities section, for the 4GB Patch exe. A lot of these sims were based on OS that would only allow 2 GB of memory to be used. With this utility, you can get them to use 4 GB. Again, this is for 64 bit systems.
http://www.ntcore.com/index.php
Here's a better explanation on what this proggy will do!
http://www.sas1946.com/main/index.php/topic,2476.0.html