Note that if your Video card has less than 4GB dedicated Video RAM, it is turned off by default. You can turn it on, but be advised that if you run out of Video Ram in the viewport it can crash. Also note that the viewport mode only uses one card or the CPU, which ever the scene fits on. 

Further note It is not recommended that you turn off the CPU. That gives your computer nothing to fall back on if you exceed your video card RAM.