Thanks for your help.
I would go for 780 STRIX 6GB VRAM.

I don't have any Nvidia's cards,so I haven't use this render engine before.

I'm thinking about this two graphic card.

1.Asus Strix GTX 780 OC 6GD5 (New,not used)
13000NTD (about 410USD)
http://www.asus.com/Graphics_Cards/STRIXGTX780OC6GD5/

2.EVGA GTX 780 Ti Classified K|NGP|N Edition (New,not ...