It goes back and forth on which is better depends on the games. From what I hear though the 6990 is faster but alot louder the 590 is just as good and alot quieter. You would only benefit Crossfire or SLI if you play at a high resolution 1920x1200 or more.
Brand & Model of your Mobo & PSU?
One better GPU is almost always the better option. Two GPU's cost twice as much, consume more power, generate more unneeded heat in the case and you get a maybe 20% performance increase in the few games than can utilize two GPU's.
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