I like SLI but given your choices I would go single GTX780..reason 1- no SLI 2- more memory.. the SLI is stuck with only 2 gigs they do not share the 780 has the 3 to use. Now I have SLI x 2 or Quad they only have the 2 gigs (bummer) but each card is faster than a standard 780 only reason. In your case the single 780 is faster than the dual 760's. Hope this helps.
Ok thanks guys, it's decided then.... the only problem I have left is either order an aurora now or try and wait until the new gen unlocked I7's are released... but that could be Sept-Oct and to be frank I don't have any patience... Also I need tbe able to play star citizen and my current old desktop is crashing all the time.