Hi there,
Yes, of course you can use any amount of ram you wish (within the restrictions of the motherboard. The type of GPU makes no difference to the amount of system ram you can use.
In terms of speeding up the GPU, it doesn't make a difference in performance that much at all. It can however, make your CPU work faster, and supply the GPU with date more quickly, which can result in a slight increase in FPS. But as a direct link to the GPU faster system ram, doesn't impact that much at all.
Edit: 16gb is a good amount for a gaming system now. The 'Sweetspot'!