The amount of RAM your PC has installed, does not affect in any way what GPU you choose. Genrelly speaking, a medium/high end gaming PC should have a minimum of 8gb of RAM, as this allows for the increased RAM usage when playing games on higher settings. A 650Ti will not play modern games at the highests settings, and so you dont need 8gb of RAM. If you were concerned about this then I would siggest upgrading to 6gb to be safe, but I wouldnt say it is essential unless you are going to be running multiple programs in the background at the same time as playing games.