Hi. Firstly you must understand that FSX is a 32-bit program which means that it will only use a Max of 4GB. So you could have all the RAM in the world and still get an 'Out of Memory' (OOM).
It all comes down to how detailed/complex the addons are that you are running and how much memory they use. Your FSX slider settings can also impact this.
Naturally, more RAM is better, particularly on a 64-bit OS that uses >4GB. Now, whilst this does not benefit FSX, the additional RAM in your system can be used to run the OS and any applications running in the background, potentially giving FSX enough free RAM to use its 4GB limit if required.
The spikes you are seeing are not RAM related. It's an indication that your graphics card is being pushed too hard. Perhaps, back off on the sliders and this may help reduce the occurence of OOM too.