$$ you'd get for selling your 780ti + $$ you'd pay for sli = total
find a gpu near your total, dooes the 780ti sli come within or do better than 80% performance of the single card? If so - go sli, if not - buy the card you could get if you sold your 780ti and used the money you put aside for the second 780ti.
>on par with a 280X now
>outperformed a 290X at launch
The Way It's Meant to be Gimped™ :^)