Objects Quality (CPU/GPU): High to Ultra, on a higher setting will reduce object pop-in but comes at the cost of frames.
Texture Quality (HDD/GPU): High to Ultra, High stops a lot of the building pop-in Ultra basically eliminates it.
Sampling : Use 100% unless you have a very specific reason.
Update to the latest beta drivers for AMD or NVIDIA to get the best performance.
Steam gives you the option to install ArmA to a different internal Hard Disk Drive, this helps to improve load times and stability.
The TweakGuides Tweaking Companion is all you will ever need to tweak and maintain Windows.
Check out the free PC Maintainance, Security and Utility apps here to keep your PC running tip-top.
Using Razer GameBooster may improve your performance.
MarkC's Mouse Acceleration Fix will let you get that 1-to-1 mouse movement you've always been after.
Enabling the 3GB switch on 32-bit Windows XP/Vista may improve performance/stability.
Use ProcessLasso (or RadeonPro with AMD GPU) to set ArmA 3 to always run on High Priority, and help steady performance.
Disable CPU Parking, to help steady performance.
Use ArmA3Mark to fine tune your settings to get visuals and performance you are happy with.
MSI Afterburner has an OSD for memory usage, when changing settings (especially view distance) try to keep it as high as possible to avoid bottlenecking, I aim for over 90%.
When changing settings your CPU may start bottlenecking your GPU.
To help tune your settings ArmA 3 has a Frames per second on-screen display in the video options, after changing a setting and waiting a few seconds your FPS OSD will catch up so you can quickly see if you get a gain or a loss in performance.
Running at the settings recommended however will yield good results both in performance and visuals.
No-one can really tell you the best settings for you as it is up to your computers build and your personal preference, this is meant as a guide only to help you make decisions.
The built in ArmA 3 Frames per second on-screen display.