RAM Questions (bitrate vs GB,etc)

GA-A75-D3H motherboard says it can run 2400(OC)/1866/1600/1333... Does the 2400(OC) mean that I would need to OC the whole thing? Llano system is known to not be compatible with OC.

I currently have 2x4GB@1333MHz. Wondering if anybody could tell me whether I would be better off with 8GB@2400MHz(if compatible), 8GB@1866MHz(failing the 2400), or 16GB@1333MHz. I'm wanting to upgrade but cost is an issue right now. Running 7 Days To Die, Fallout 4, other games that run fair-to-middling at low settings. For instance in Fallout 4 I have certain mesh settings turned down to the point that most railings and brush outdoors is not rendered unless scoped aiming. 7 Days to Die is set to half textures with all boxes unchecked and I get 18-22FPS near a town, closer to 30FPS in wilderness areas. 7 Days is a very RAM intensive game due to voxels, which I have been able to mitigate with Readyboost and a LARGE paging file (16-32GB), combined with OS and games on separate HDD's.

better off with 16GB@1600MHz for DDR3 RAM. If you can get higher clocked RAM for cheaper, do so; don't OC (overclock) it unless you know what risks you're going against. never disable the pagefile, though, no matter how much RAM you've got!

for DDR4 (which requires a diff motherboard), anything over 3000MHz (including 3000MHz) is suitable.