It's recommended to always spend in a stronger card rather than adding a 2nd one to do SLI/Crossfire. The performance increase with 2-way SLI/Crossfire is not always the same (up to 70% more, 50% average) and sometimes you may loose performance. The driver needs to have a SLI profile for the game you want to play, if there's no profile the game will use only 1 GPU.