Direct X is a set of APIs, that allow developers to directly code their video games to use the hardware for specific functions. It also allows other developers of applications to enchance the experience with technologies like OpenGL and the such to make cool animations, and 3D GUIs.
http://en.wikipedia.org/wiki/DirectX
And no, over clocking is highly pointless in many many cases, it lowers stability of your system and causing more damage over time to your hardware. Its like tossing NOS in your car, your engine block ain't bullet proof and you keep pushing it that hard, it will fail. Heating up your hardware over and over again like that ends its life faster and faster.
As for real world performance, you will get some but you won't notice it in real time. I mean, there is no way a human can tell the difference between getting 70fps and 77fps in a game.