You are missing some kext to add support for the Geforce 8400M, without that the graphics are not hardware accelerated.
You dont need to reinstall, you need to learn to how to install a kext and find out one that works with your geforce card (I would try nvinjectgo or natit) and install it.
Keep in mind a non working graphics kext can stop your system from booting so you should better learn how to recover from that (booting in safe/single mode and deleting a non working kext).