Can you tell me how YOUR needs will change?
I think even a single GTX780ti will run games very well on a single 2560 x 1600 monitor.
If you will game on triple monitors or a 4k monitor, all bets are off, and dual cards of GTX780 caliber will be needed for best gameplay.
On vram, I am dubious about the need for lots.
Here is a comparison of 2gb vs 4gb.
http://www.pugetsystems.com/labs/articles/Video-Card-Performance-2GB-vs-4GB-Memory-154/