I gather this is for 4k. Else its a waste of time and money.
One R9 295 is a bit better than 2 x GTX780Ti in SLI
So 2 x R9 295X should be better than 4 x GTX780Ti in SLI.
You can really only do 4 cards with X79 anyway. But then again it might pay to have an i7 4960X pushing them along.
And you'll need 1600W of power supply for either.
And a bloody big case. I'd say this'd cost you $5k to $6k all up (without peripherals)