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Overclocking
Superclocked as they might be, these XFX cards surely have some headroom left under their hoods. Rather than bore you with a verbose description of our overclocking attempts, I'm going to paste my notes verbatim. I overclocked using AMD's Catalyst Control Center application, tweaked voltages with MSI's Afterburner utility, and tested stability with Kombustor, another MSI app. (XFX doesn't provide its own overclocking tools for these Radeons.)

Ready? Here's how the 7850 Black Edition fared:

Core:
1000MHz -- OK after 5 min burn-in
1025MHz -- OK after 5 min burn-in
1050MHz (max) -- OK after 5 min burn-in

Memory:
1250MHz -- 6506 kombustor -- stock
1350MHz -- 6925 kombustor -- ok after 5-min burn-in
1400MHz -- 6690 kombustor -- reverted
1375MHz -- 7072 kombustor -- ok after 5-min burn-in

Overvolt: no go. afterburner hard-locks system

Max: 1050/1350 (vs 975/1250 stock).
Temperature around 75C during kombustor burn-in.

And here are my results with the 7870 Black Edition:

Core:
1200MHz -- OK after 5 min burn-in
1300MHz -- crash
1275MHz -- crash
1250MHz -- display coruption
1225MHz -- crash

Memory:
1250MHz -- 6721 kombustor -- stock
1350MHz -- 7194 kombustor -- OK after 5 min burn-in
1400MHz -- 6959 kombustor -- reverted
1375MHz -- 7354 kombustor -- OK after 5 min burn-in

Max at stock voltage: 1200/1375 (vs 1050/1250 stock).
Temperature around 87C during kombustor burn-in.

Overvolt:
1300/1375MHz -- 1.300v -- crash
1275/1375MHz -- 1.300v -- overheat
1250/1375MHz -- 1.300v -- overheat (over 105C in kombustor)

These results are a little disappointing. Neither card seemed to tolerate overvolting; the 7850 Black froze when we launched MSI Afterburner to tweak voltages, and the 7870 Black overheated quickly, with its GPU getting hot enough to boil water and subsequently shutting down. We'd excuse that behavior if the settings we tried had been particularly daring, but our reference Radeon HD 7870 handled itself just fine at 1250MHz with 1375MHz memory and 1.30V.

Clearly, the AMD cooler is better equipped to handle high overclocks than its XFX Double D counterpart. Our findings suggest the difference may hinge on the amount of metal under those cooling shrouds. Fan control didn't appear to be an issue—the XFX 7870 ramped up its two fans without fault as temperatures climbed, hitting a speed of 73% at 95°C and 100% at 101°C. After that point, though, the card started throttling itself to stay cool, even with the fans pegged.

Happily, when overclocked by hand, the 7850 Black emerged out of its torpor in Battlefield 3. It outran the reference-clocked 7850 handily, nestling itself not far below the Radeon HD 7870. Load power consumption only increased by about four watts, as well, which is negligible.

As for the 7870 Black, even though its overclock didn't match that of our reference card, it still put on a good show. The temperature we measured in our Kombustor burn-in (87°C) makes us a little wary of applying the same overclock in a cramped PC build, though. Honestly, if you're going to overclock a 7870 at all, we'd recommend getting one with a more capable cooler than what XFX provides.