I know some people with switchable cards have been having issues but I can't really remember what they did. Did you double check if indeed the powerful one is used ?
The game is also notorious for having high frame rates so on powerful cards, if you don't limit them i can lead to problems as well. Try checking on them with the fps on cheat. IF they are over 80, you need to limit them either by using a fps limiter program (there's one especially for the Sims) or by setting the vsync on your nvidia settings.
Some more tips on crashes and lags are here:
http://answers.ea.com/t5/The-Sims-3/FAQ-Game-Grashes-CTD/td-p/656/jump-to/first-unread-message
http://answers.ea.com/t5/The-Sims-3/FAQ-Lags-amp-Freezes/td-p/646/jump-to/first-unread-message