Firstly, I'm not sure if you mean ONE 4870X2 or TWO 4870X2's, and if you mean TWO of them, I'd suggest getting 2-3 HD 4890's instead, as they'd be cheaper and faster.
Second, we need to know if you're going to overclock the 965, though at stock it should be fine.
You will be fine with a single 4870X2. If you want to add a second 4870X2 for 4-way crossfire then your CPU does begin to become the limiting factor but of course overclocking it will help.
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