Is HD5870 bandwidth starved..? these games where 4870x2 is faster suggest so:
FO3 82.5 72.3 14%
FC2 75.6 66.5 14%
RE5 124.9 98.9 25%
BAT 131.9 108.2 22%
HAWX 105 74 42%
5870 is double everything, so just as 4870x2, although not handicapped by lower 750Mhz and CF efficiency. Only explanation is x2s 230GB/s vs 153GB/s bandwidth.

Solution? 6Ghz GDDR5 chips would provide 184GB/s - that extra 20% should close gap. However, to get 230GB/s would require 7.5Ghz GDDR5... AMD probably left room for the 5890!

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It is very very doubtfull that nVidia would go from 384 GDDR3 -> 512 GDDR3 -> 256 GDDR5 and end up with similar bandwidth limit issues and thus barely faster than GTX285. Highest GDDR5 chips in spring 2009 (when first cards fabbed), are 6-7Gb/s from Samsung. Thus, both nVidia and AMD probably have either 384 (1.5GB) or 512bit (2GB) parts by early spring '10!!