Wow, 16? Do you know what speed it's running at? Are you using a 64 bit operating system?
Here's what I'd do. Open the Windows Task Manager, then load yourself up into a flight with FSX in window mode, so you can see your physical memory usage in the bottom right corner of Task Manager. Then taxi around, or do whatever you do when it normally freezes/stutters for you, and watch your physical memory usage. If it's maxing out near 100%, then that'll help narrow things down. If it's not, then it might be something else.
Testing each stick of RAM might be a little tricky depending on your system. Essentially what you'll do is remove it from your system, and try booting your computer up each time with a different stick in. If your computer fails to boot on one of them, then that stick of memory has problems. The tricky part is remembering your RAM could be fine, and it could be a DIMM problem. So use the appropriate DIMM at first, and test all of the RAM. If the computer boots up and behaves without blue screens on all of the RAM, then you test each DIMM slot on the motherboard. If the computer fails to boot with one of them in use, then it's your motherboard. There's also other things you could do, like doing custom Prime95 blend tests with 80-90% memory usage, but there's countless ways to go about doing all of these things...and I don't really want to be held accountable for things going wrong.
For right now I'd suggest doing the Task Manager/FSX test.