More then likely you can't OC that Dell, Even if you found a wat, You might squeeze 100mhz out of it if your lucky.
That i7 you got is perfectly fine, Get your self a better Video card and you will see an good FPS improvement.
If your not totally sure what video card you should get, Start a new Topic and ask and mention what your budget range is, and Powersupply is, and I know 1 of the member here will lead you in the right direction.