well from my knowledge... If you are going to run SLI the first card gets 16x and the other either gets 1x or 4x something like that. Depending on the mobo specs you might not even want to go SLI, who knows. But i only found solid specs on the 7900 its looks good to me. they both have ddr3 mem and the pixel per clock might be alittle higher on the 7950.