Personally, I would like to upgrade my RAM either 16GB or 32GB. They say 16 is minimum, but 8G is enough for most users. If running at 32GB of RAM, I think that would be enough for most 3d gaming in these days.    

 

 

I'd say upgrading your CPU would be a top priority for flight simming, especially the FSL A320 which requires quite a bit of behind-the-scenes calculations which does impact performance. Assuming you already have 8+GB of RAM, you should then focus on upgrading your GPU and overclocking both before you focus on RAM. As you said, 8GB is enough for most, though personally I think 16GB is best to give enough buffer room. 32GB is overkill for anything except heavy-duty video rendering.