for best Arma performance (in my opinion) upgrading priority is this:
- CPU - heavy affects frames for arma
- GPU - will get you better frames
- RAM - need at least 8GB of ram IMO (faster=better)
- Motherboard - If your running a 7-8 year old motherboard, then that may be part of your issue. What good is a super fast GPU, CPU, and RAM if the bus lanes can't support the max component output?
- Hard Drive - SSD is always better then hard disk... putting in an SSD is probably the easiest / cheapest upgrade, but will likely not get you any better frames, only faster load times...
- Power supply - I only list this because PSU's are an odd animal... they don't just work / not work and are very hard to troubleshoot. Just make sure you have enough wattage to handle what you have. (also if your game crashes during times of heavy load and you've ruled out your GPU, PSU is a possibility of root cause...
Hope this helps, of course this is speculation as Arma is an odd animal when it comes to graphics...