Though your motherboard has SLI which is a Nvidia format for twin cards you certainly can put 1 AMD card in one of the slots wuth no problems.
Remember to remove the Nvidia drivers for your current card before installing the drivers for your new card. Its better to go and get the latest drivers from the amd website as your driver disk that comes with the card will almost certainly be out of date.click here
I have the the 6870 and am very pleased with the results in gaming etc.
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