6 GB 780? Who told you that?
"UPDATE: According to EVGA, plans for GTX 780 with 6GB memory are from July, but this card was never released and most likely never will be.
ORIGINAL STORY: EVGA is slowly preparing for GeForce GTX 780 TI launch, but of course the GPU industry does not centre itself around one model. That said, older cards will see a refresh."