I too suffered from this on my sims games... I used to have only 768MB RAM and a Nvidia GeForce FX5200 Vid Card... soon as I upgraded my RAM to 2GB and my Vid Card to a 512MB Radeon x1650, I've never has an issue with graphics anymore... I'm no whiz at PC upgrades but I can suggest checking
Crucial.com for the RAM... they have a tool that will help with selecting RAM, and for the vid card you will have to check your PC's specs to see what kind of slots you have... whether it's PCI or AGP.... also you might want to look into getting more HD space as well. Hope this helps.