Will an AX 860 be enough to power a 4770k along with a gtx780 Ti in sli with gtx750
Cant do SLI between those cards - needs to be 780ti + 780ti. Plus the 750's cant even be in a SLI setup (no bridge). Motherboard must also be SLI compatible.
By all means have a 750 for physX if you really want to waste money. Apart from that there is no reason whatsoever to have it in a rig with a 780ti. The majority of 780ti's have enough outputs for several montiors.
AX860 is enough for a 4770k + 2x 780ti's + everything else + with 200w to spare.