AFAIK, CX is of lower quality than the other TX and HX PSU's. Go for the TX or the HX if you can. The TX 750 watts will be enough for the 7870's, IDK if it'd be enough if you extremely overclock CPU and/or GPU's.
If you're not sure of crossfiring another card and will continue single route, then the Corsair TX650 watts should be more than enough.
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