A 970 with an FX41 lol.
DayZ will be hammering his CPU.
The FX4100 really is a poor CPU. I'd never buy one in 2011, let alone use it approaching 2015!
A bit of mild overclocking would sure help out that out. This is just what I would do though.
If one day he was to play another game instead of DayZ he would be set, rather than having to upgrade again.