Help me : crossfire path or nvidia?

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iam using : msi p45 platinum, e8400, 4850, 2x2 viper & v450vx and experiencing stuter / delay on games : Company of heroes & dow 2, i had no physic, 2x AA & all med to low but the texture, should i add another 4850 or sell it & grab 9800gtx+? and does my psu able to handle crossfire?, thanks

 

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currently iam using 1680x1050, thank