Sorry if it offends, but it sounds as if you’re throwing money at a problem that you don't understand.

First off - HAVE YOU MADE A BACKUP? Do that before anything else. There IS a reason that you need to do this first.

If you have Windows, have you made a "System Repair Disk"?

Next - download a free program that shows what is going on in your computer. YES. I know that you can press CTL/ALT/DEL. But some viruses will pause their activity if you do that. What you want to look for is how active your CPUs are. Are they all hitting 100%? How much activity is going through your network card? Also check your hard disk activity. A lot of reads and writes? If so, you'll have to read up on it yourself. Too long to go into here.

If you think that you might have a virus, MAKE A BACKUP. Even if your backup now has a copy of the virus you still have a backup. I had a virus move the MBR (Master Boot Record) to a new location, AND it encrypted the hard drive. After I removed the virus I could no longer start my computer and no data was recoverable.

In so far as the computer crashing you could be having a component failure: hard drive, motherboard, memory, even the video card could make your system crash. No matter how new your system is, you can still have something go bad after a bit of use. Is the fan on the CPU running? What IS the CPU temperature?

Unless you're editing movies, your memory is more than you need. But as you have so much, you have 2 ways of testing it. The best is to download one of the many free memory testers. If you find one that doesn't require you to restart you computer to test it, then skip that one and find another one. Generally your looking for one that will autoboot from a CD BEFORE your operating system, so you may need to change your BIOS. The other way to check your memory? Just swap it out, one at a time. Not the best way, but it often works.

The question "could it be too that I need a more powerful video card?" is what makes me think that you're just throwing money in the vain hope of fixing the problem. I have a Radion HD 6870 (a much slower card compared to yours) with only 1GB of DDR5, and I usually run 5 to 6 monitors with most of the settings in FSX near the max settings.

In a nut shell - your problem probably isn't in the flight sim. That's just where you're noticing it.

You should be running FSX very good!
That being said, the question could be, how much programs/startup/processes you have running along with FSX, ref your memory problems.

You could try
http://game-booster.en.softonic.com/
Just to see if it helps.
Good luck.