It seems that it works best with 2 of the same and some write you can find your high end GPU dragged down by pairing with too low a model instead of gaining. I'd remove the 4870 for power reasons alone.
So I have an old Radeon HD 4870 x2, and I was wondering if I was to get a Radeon R9 380X, would I be able to use crossfire between them?

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