For that price range you have 3 options:
On nvidia side you have the GTX780 and o AMD side you have both the HD7990 and the upcoming R9 290X.
On raw power the 7990 is the winner but as it is a multi gpu solution it can suffer from issues like stuttering, you may want to check reviews of the games you're interested to see how it behaves.
On the single GPU solutions, the R9 290X is supposed to be on par with the Titan, but as it is not released yet we have that for certain, although we do know for a fact that the GTX 780 is a monster and will most likely max everything.
TL;DR: Get the 7990 after checking the titles you're interested works fine, otherwise, get the 780.