no, you cant use those 2 cards, i would only use the 6800gt since its much better. The only way you can use 2 graphics cards is if they are SLI ready (6600gt or newer i believe), adn they need to me the same type of card (ex. 2 6800gt's). If you put both of them in i doubt you will be able to see any picture, so i would only use the 6800gt.