I am hearing a lot about how the 7850 has a ton of OC headroom. I've heard it said you can OC a 7850 to perform like a 7870. But if you are willing to OC the 7850, wouldn't you also try to OC the 7870? So assuming you do a conservative, realistically possible overclock of both cards, how does the gap of performance look?
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If you purchase the MSI R7850 PE with around 1.25-1.26v 1260MHz easily! Just make sure you got good case with good airflow.
How does 1260Mhz translate into 7870 terms?
Also that specific card is only $10-20 cheaper than a lot of 7870 cards.
The 7850 hits 1200+ which is about as fast as the 7950.
The 7870 can hit 1250+ so it puts it slightly slower than a stock 7970.
Generally you get more performance gains from a 7850 but the 7870 OCs pretty well too.