Those spikes are just one symptom of running OOM, best is to check the actual usage of memory with Task Manager or there are more programs that can do this for you.

 

System optimization is always best if you want to get the most out your flightsim and it takes a lot of trial and error. I recently built a new system and with several years of experience it took me 3-4 weeks to get it optimized. And I still have some minor issues I'm still working on. A flightsim isnt like a standard game which you install and start playing.

I think the best start is to do a full reinstall and optimize as much as you can, then start FSX with the lowest possible settings and see how far you can push settings while keeping FPS high enough and from there you have seen what every setting does, what should enable you to come to more specific questions.

We cant see the details of what your system does, so we can only give general advise and the more specific the questions get, the more we can go into detail about fixes for certain problems.

Is there like a way to give FSX more memory? or should i buy more memory?

Unfortunately not. FSX is a 32-bit program and is limited to 4GB. Some extra memory is good, so there is a little more room for other programs that are 64-bit, but we all have to organize how we divide the use of memory by turning addons on and off.

But FSX without addons should stay well below 4GB, depending on how you optimize your system.

 

I work with P3D, but that has the same basic structure as FSX, and I use for windows alone about 1.2GB. With P3D running without addons, I'm using about 2.7GB (not sure what FSX does in this case, but it shouldnt be much more then 3GB) and from there I've got about 1-1.5GB I can use for addons.

If you're already going up to 4GB with only windows and FSX running, so no addons, there's definitely room for improvement, starting with Windows itself. This can be done by turning off unneeded programs and services that are activated at startup. I think this is also discussed in the forum post I linked, from NickN.