Arma 3 server side is very CPU intesive, even without AI. Clock Speed is always a problem. Also, I think the minimum OS for Server is Windows Server 2008 R2 because its the Windows 7 equivilant which supports DX11, Server 2008 is the Vista equivilant.
Minimum Clock speed should be around 3GHZ, you can get by with less but more people the lower the frames. You could probably get by with a Core i3 Dual Core at 3GHz+ if you only run one instance and is dedicated to Arma 3.
I run a 2.5 Ghz Dual Quad and it runs about 25-30% on 40+ players, but only uses 2 cores. Frame is an issue though, so I'm trying to find a cost-effective dual core high clock server.