GPU decition.

I just ordered a PC on a budget of 1600$

I enden up with a ZOTAC GTX 570 1280mb and i was wondering If it would be good to get a second 570 or just go for a single 680?

Powersupply: 620W,CPU: i5 2500, Motherboard: ASUS Sabertooth P67, 8gb ram 1600MHz.

Edit: the ZOTAC GTX 570 is already with it so i would have to just throw it away if i got a 680.