Quick question. Are you all or some of you running the bare minimum requirements? It would help more if you listed the actual specs of your computers. Remember the game has listed the "requirements" for the game to run, but they do not say "FANTASTIC game play with this minimum requirements".
My suggestion to those that have at least 2GB RAM, Dual COre Processors, and 512MB Video (that works with SIMS3)
You may have so much starting up on your computer taking up half of your RAM from the start. Are you running Vista? XP? Those two will play a major role when it comes to sims3 play and your Memory avail.
I highly recommend you check out this free little tool (its far better than any System Task Manager by far) to see what actually is running on your machines, and what you can or cannot shut down on start up or while running the game.
I hate that fact that all these software companies think we want their little piece of heaven to load on each system boot. Its annoying yes, but we as PC owners have to keep it under control. Find out whats running and much more by this free tool.
http://www.gtopala.com/ Get the Standalone English Version (No Need for Pro) or get the multi language if needed.
Once you investigate a bit more to find out what you can safely shut down you may find Sims runs better. Another GREAT place to review is Black Vipors helpful settings. He tells you what you can safely shut down in the OS and more. Like what is the default, and if it is needed and why. I suggest you choose your proper OS i.e. XP, Vista ... this is the link to the XP SP3 32 bit
http://www.blackviper.com/WinXP/servicecfg.htm
GOOD LUCK everyone. I know how much you want to play the game so I figured I'd take the time to at least offer some help :)