Firstly, how much are you trying to spend?
Do you live near a www.microcenter.com?
You could always just sell your current mobo and pickup a 2500K and z68 mobo from microcenter for around $350
Then get a new 850W 80+ certified bronze or higher psu and a 2nd 6970 and you are set with zero bottleneck.
Of course this is if you were to ever want to max games.
For what you are trying to do right now, you are best with just getting a 960Thttp://www.newegg.com/Product/Product.aspx?Item=N82E16819103995
It has a chance of unlocking to 6 cores. It is closely similar to the 955BE which was the best budget cpu before they stopped making them.