The 780ti is faster (but not my much, at least from what I have seen) but it is more expensive than a 290x (I don't think the 290x is really worth it over a 290). If it were me, I would buy either a 290 or a 780 and pocket the change for a later date. Grab a decent CPU cooler and over clock you CPU while you are at it. That should keep you fixed for performance.
Crossfire / SLI 290's 780's is also an option, but depends on what board you have. Not sure on the PSU whether it would be fine for dual card due to its age.