I believe your problem might be the version of DirectX you have installed on your PC. All current nVidia cards are not compatible with DirectX 11, even the top of the line $500 cards.
My suggestion is that you uninstall DirectX 11 and Sims 3. Then install Sims 3 and DirectX 10.
I can't think of one game out right now that uses the newer DirectX 11. So there is no need to install it even if your card supported it.
RE: Sims 3 bizarre issue!
I believe your problem might be the version of DirectX you have installed on your PC. All current nVidia cards are not compatible with DirectX 11, even the top of the line $500 cards.
My suggestion is that you uninstall DirectX 11 and Sims 3. Then install Sims 3 and DirectX 10.
I can't think of one game out right now that uses the newer DirectX 11. So there is no need to install it even if your card supported it.