Thanks for the replies, I am also concerned why my game stutters in some games, for example San Andreas. When I'm walking around it is fine but when I'm in a car it stutters like hell. Does anyone know any fix to this? And also I'm running Windows 8.
Its just the way DDR memory is represented. The first D basically stands for double so you always double the clocks speed because DDR memory transfers on the raising and the falling of the clock signal, so 400mhz ram as is the true speed, effectively operates at 800mhz because it can transfer double the clock speed, or effectively at 800mhz.
Nothing to worry about, just the way GPu-z reports the speed.