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The 7990 is not an official AMD release and has many issues,the 690 on the other hand is much more hassle free,especially since your building multiple rigs.
i believe the Mars 690 was never accually released,they had prototypes of it,but thats about it.
as for performance the 7990 should handle those high resolutions fairly better than the 690 but again it has some issues(overheatings,far from perfect crossfire drivers)
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