Performance issues can be boiled down to a few things, but ultimately the game is broken on a fundamental level with memory leaks out the ass, and exponentially increasing memory leaks for every added expansion pack.
>Game is 32bit, it can only utilize 3gb of ram max on a 32bit OS, 4gb of ram on a 64bit OS, but by default is set to use only 2gb.
In the sims game directory
Program Files/Electronic Arts/The Sims 3/Game/Bin
open Sims3.ini and set
MemoryUsageLimit = 20000000
to
MemoryUsageLimit = 40000000
This mildly improves performance.
Another issue is the framerate, if framerate is not locked to 30fps, it tries to render faster and hinders performance.
This resource should prevent it from trying to murder your graphics card:
http://www.moreawesomethanyou.com/smf/index.php/topic,15585.0.html
NRAAs are a must for cleaning up the garbage and broken AI.