My specs - The basics anyway...
Intel i7-4770 CPU @ 3.4GHz (Not overclocked or tweaked in any way)
Nvidia GeForce GTX 760
16GB RAM (Don't remember the brand but it's new within 4 months) - the system came with 8GB - the big improvement I saw when adding more ram was time for rendering videos dropped dramatically. Not much improvement in Arma.
Windows 10 Home
I actually bought my system thru Newegg a year or so ago for about $900 and been super happy. I've got nice peripherals and do recording and editing without any problem performance wise.
I've also tried to optimize Arma but it seems that individual experience is very dependent on the server performance. During most Ops I get about 30-50fps but when the server is chugging it will drop to 10 easily. From my experience, if you're mostly playing Arma, if you just have a decent processor and a decent to good video card with at least 8 gigs of RAM you'll probably get as much as Arma can give you. Again, just my experience but I don't know if you need to go all crazy on hardware as Arma just isn't optimized that well for multiplayer....in my opinion.
-Brewer