Hi and welcome! From reading other build threads (since I am not an expert), OverClocking doesn't help as much as actually getting a stronger GPU than most think. The GPU you chose AMD Radeon HD 7870 is about on par to a GTX 660, 660Ti according to this chart:
Graphics Card Hierarchy Chart - Best Graphics Cards For The Money: July 2013
Since your dropping quite a bit of money, you want to focus more on the GPU than anything else unless your doing something else besides gaming.
I would personally say get rid of any OC stuff (unless that's what you personally want) and focus more on that GPU.
Also, have you considered building your own PC instead? It would save you a TON by making it yourself and allow you to get a higher grade GPU.
(Someone else will probably come with a more in-depth post ^_^)
edit: Here is a thread regarding CPU's and gaming. Very informative. It also coveres HDD's and SDD's which I forgot to mention. Get an SSD, while it doesn't impact your gaming in a FPS type of way, can make game loading times much faster as well as program and boot up times.
Your CPU and modern games: A guide to those building.