Switching between Intel and NVIDIA GPU

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http://askubuntu.com – My laptop has 2 GPUs, the Intel HD Graphics 4000, and NVIDIA GeForce FT 650M. How do I switch between GPUs? I tried installing drivers from NVIDIA, but that ruined my Ubuntu installation, and I had to reinstall. The Additional Drivers in Settings don't list any Nvidia drivers either. (HowTos)