My personal feeling would be more along the lines of the basic i5 (4440?
IIRC) with a H87/81 MoBo.
Do you need to overclock? Do you even wish to? I'm running mITX and am now extremely pleased with my system (in my case the i3 does all I desire, it was a good price, runs all my games and most of my code is single threaded or GPGPU - so a no brainer for me). You do have to consider heat much more if you go mITX, you're not going to get a crazy overclock unless you watercool anyway, (even with an AIO, case options are severely limited).
Pob said to me in one of my threads a while back (paraphrasing): ITX is all about choosing your compromises. It took me a while to get this; you can't balance heat to noise in ITX the same way as you can in a larger case. For me, if this means I'm not overclocking, so be it... in time I may pick up a 65W i5/i7, but I'm not going to stick a higher wattage CPU than that into my case, because this would require a compromise on either temperatures or noise that I'm not willing to make.
You say Arma runs fine on an IB i7, but not on the Phenom II X6, this implies that it must be fairly clock speed (and of course IPC) dependant and probably doesn't require 8 threads. If 4 suffice then I'd be inclined to go with a lower i5, the difference between all the i5s (non OC) is
IIRC less than 10%, for a much larger increase in price.
You do pay a premium to have a happy life on ITX - things like cooler compatibility is still much harder and you frequently end up paying more for less material in a smaller form factor (i.e. to keep my i3 quiet I stuck it under a Noctua L9i - this cooler costs almost double a Hyper 212 and has lower performance).
Ok I seem to be rambling a little now - maths exam today has possibly messed up my mind slightly...
My opinion would be a low end haswell i5, with a H87/81 (depending on features) motherboard, in a CM elite 130. I went through a similar intel vs. AMD debate to you when I bought my rig and nearly ened up with a 760k, but my decision was made due to deciding that a 760k would need OCing => more expensive MoBo and a better cooler (like a H80 to be able to fit it into ITX). Whereas intel was a dear CPU with a cheap motherboard, no OC required. For me that was a self-answering question when I really thought about it.