Contrary to popular belief the RAM on a graphics card does not make it go faster.
Below is a chart showing the difference between 1 Gig GTX650 and a 2 gig GTX 650. The performance is based on 3Dmark11 and Unigine Valley & Unigine Tropics benchmarks. As you can see the 1 gig card actually scored better than the 2 gig card.
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.
As a general guideline.....
The GTX 650ti is approx 50% faster than the GTX 650
The GTX 660 is approx 40% faster than the GTX 650ti
The GTX 760 is approx 30% faster than the GTX 660
The GTX 770 is approx 10-20% faster than the GTX 760
The GTX 780 is approx 10-15% faster than the GTX 770
The GTX 780ti is approx 10% faster than the GTX 780