Mini upgrade - should I keep current 1150 CPU, or not?

Hey all,
Brought over some bits of my old PC to Canada;
CPU: 3.4GHz 15-4670K
RAM: 8GB (2 DIMMs of Corsair, forget actual timings offhand)
GPU: GeForce GTX 760
SSD: 120GB and 500GB
Mouse: Razer Deathadder 2013

Shall be using it for mainly ARMA3, and photo processing, with the odd video encoding.

Wondering should I stick with the current quad i5 CPU, or should I upgrade? Shall be needing a new mobo, and would prefer to stick with the current so I'd only need to get a mobo and two more sticks of RAM (to bring it up to 16GB), along with a case and PSU.

Don't need a Windows license as have a legit license, and shall be using some 19" pos monitor until I can get a cheapish 24" monitor.

Currently have no gaming PC, so prefer to get this up and running, with the possibility of GPU upgrade at a later date.