You and me both quite honestly, but the truth is this information has not been disclosed by the slightest so far by Bohemia. I frequent their forums quite often and the implications so far point to a similar architecture of that of ArmA 2. Upon that theory, you should be good with a quad-core depending on model and of course, brute gigahertz.
The reason I mention the alpha, is because it is the most logical (albeit, general) answer anyone can give you at the moment. So until more details are revealed concerning the potentiality of how much ArmA 3 utilizes multi-core CPUs and hyper-threading technology (something ArmA 2 doesn't play nice with), the alpha will be the only way to gauge what is what.
If not from your end (because of hardware limitations) but by the experience of others out there with such hardware, which I'm sure people will share their experience/results concerning performance. Its (performance) what everyone is looking to find out right now when it comes to ArmA 3.