How much is it overclocked ?
I suspect you're looking at 2 GPUs with similar prices.
Let me help you with that.
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Here are some comparisons
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As you can see they both have similar performances. Even if you OC the HD 7870 , it would still beat the GTX 660 ti.
The HD 7870 comes with 2 Free Games while the GTX 660 ti only comes with batman arkham origins.
So of course the HD 7870 is a better deal