Crossfire 7870 bottlenecks

So I'm planning on buying 2x HD 7870 GPUs. One will be the Sapphire retail-OC version, and the other the Gigabyte retail-OC version and going crossfire. My motherboard will be either the ASRock 970 EXTREME4 or ASRock 990FX EXTREME3. My PSU is a Cooler Master M620 Real Power Modular - 620W. For CPU I'm thinking of buying an AMD Phenom II X4 965.

Now, first of all, I'm wondering what the differences between the two motherboards are, and how much of an impact they will have. Secondly, I'm wondering if the CPU will become a huge bottleneck? I believe it could be overclocked a bit without issues, but I'm wondering if I would have to really stretch it a lot to see any real benefit from the crossfire setup? Thoughts?

Thank you in advance.