According to Jetline systems, RAM is the second most important thing you should consider upgrading for your FSX build - http://jetlinesystems.com/performance.php

 

Do you know what kind of performance improvements (and by what percentage) you can gain from going to a higher MHZ ram speed, and lower CAS latency? 

 

Thank you.

 

 

2133 seems to be the sweet spot as much higher the CAS increases counter the increase in Mhz. I went from 1600 cas 9 to 2133 cas 9 and improved 2-3 FPS, but did not see an increase going to 2400 CAS 10.

 

I would say stick with at least the 2133 ram 8/16Gig and you will be fine.