Contrary to popular belief the RAM on a graphics card does not make it go faster.
Below is a chart showing the difference between the GTX 980 4Gig, GTX 970 4Gig, Titan X 12Gig, R9 390 8Gig and R9 390X 8 Gig.
The performance is based on PassMark - G3D Mark - click here for full chart
In the above chart, the higher the score the faster, and as you can clearly see the 2 nVidia 4 Gig Graphics cards, GTX 980 & GTX 970, are faster than the 8 & 12 gig cards!
The thing that makes a graphics card faster is the GPU (Graphics Processor Unit) not the amount of RAM. If you want more speed go up to the next graphics card.