Before I answer your question, let me ask why you are "upgrading" from the 6400 to the 6700"k" if you're planning on overclocking?
First, there isn't a reason to get the "k" versions other than to overlcock.
Second, if you're exclusively a gamer, i.e. not looking for a workstation/rendering machine, you won't benefit much from the hyperthreading from the i7.