That's low
I was over-doing it a bit (matching may have been better) but the 6950 should easily match the 5870 in raw power (it would only take 1280 ALUs) and by the slide the arch has gone through plenty of enhancements.
My bet is that the difference between 6950 and the GT580 may not be significant (between 0-5% +/- is not significant for me when so many games already runs at crazy high fps, even 10% and more in some cases/games is completely transparent to the end user).
I'm confident that the 6970 will beat the GT580 ( I'd put the odds 80%). Antilles should offer a "useless" amount of power
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