AMD Six Core 4.0GHz Overclocked Barebone PC - ASUS M4A89GTD PRO Mobo, AMD Phenom II X6 1055T, 8GB DDR3 1600MHz RAM, ASUS Nvidia 660ti Cooler Master HAF 922 Chassis, 750W PSU, CoolIT ECO A.L.C. Liquid Cooling......... can i keep adding another video card lets say a nvidia 670 in SLI? should i change the mobo, ram... etc?