I have exactly the same problem, but I'm not willing to throw this laptop away when it comes to gaming.
This is my setup:
Dell XPS 17 laptop with Nvidia Optimus technology (integrated Intel GPU für standard apps, extra Nvidia GT 555M GPU for games)
Windows 7 Home Premium 64 bit w/ latest updates
No matter which Nvidia driver I use (latest version from Dell homepage, or latest version from Nvidia homepage), the problem stays the same:
Even if games are setup to use the high performance Nvidia GPU in the Nvidia Controlpanel, games use the slow Intel GPU. (as shown by the GPU indicator tool)
I don't want to give up on this, because this Dell laptop simply was too expensive to just use it as an office machine. And it'lll make my wife cry if I won't get Civilization and Portal to run properly again!
Any help, hints, ideas are greatly appreciated!