Two things:
The engine from which Arma runs (the foundation of the MOD) is inherently buggy and poor optimized. The reason why I continue to come back is the incredible immersion and attention to detail. Now, that is Arma by it self. Then Mr. Hall develops a MOD on an already poorly optimized engine, which again means performance will drop.
I have a decent PC and a completely new one, with i5-4690k, GTX970 and 8GB of HyperFury RAM, and I still get poor FPS, because of the reasons stated above.
Now, the hope for SA was that Mr. Hall, would specify the Arma 3 engine to the SA needs. However, as we both know, that has been no great success. This is
not to talk Arma, MOD or indeed SA down, I love and play all of them, but what I have stated is consistent with my experience - also experience from folks with much more powerful PC than I.
Now to the second thing. In my opinion upgrading a PC based on wanting better performance from the mentioned games is flawed - it will not improve much.
If you, however, want an
overall better gaming experience, granted that you play other things than Arma and SA, an upgrade could be done and will improve the
overall quality of your gaming experience.
I am sorry that I can't give you the answer that you seek, perhaps others on this forum knows exactly what can be upgraded to make your specific experience in the aforementioned games better.
Alternatively, you can give this guide a peak:
http://dayzintel.com/dayz-guides/dayz-standalone-performance-guide