first, let's be clear, neither the 780 nor the 780 Ti is faster than a Titan at stock, when overclocked though they can match it
that said, a 780 is now $500, while the Titan remains at $1000, and even though is faster, the difference is only 10-15% and doesn't justify double the price
the difference between the 780 and Titan is so small that a 780Ti doesn't justify $700 either,
So ur best bet would be to get 2 780s in SLI for a very good cost/performance ratio