can i run fsx?

You mean Microsoft Flight Simulator X? That's a Windows program. To run a Windows program on Mac you need to install Windows on your Mac, using Boot Camp. Boot Camp is a feature of OS X. Here's Apple's Boot Camp support page: http://www.apple.com/support/bootcamp/

Of all the specs you've quoted for that iMac, the one you don't mention is the version of OS X it's running. However, considering those specs, it's going to have a version of OS X that includes Boot Camp.

In addition to Boot Camp, which requires that you sign out of OS X and sign into Windows, there are several paid apps that allow you to run Windows apps while staying in your OS X session: Parallels, and VMWare Fusion (you'd still need to buy Windows as well).

 

It's called X-Plane

X-Plane is a simulator for both PC and mac and is still supported (FSX is only for Windows and is no longer supported). From what I have read and heard as well as from what I have tried on the demo of X-Plane 10, is that you buy FSX if you want a flight game, and you buy X-Plane if you want a simulator.

Why not to the free demo, it gives you all aircraft (and will still let you install more aircraft on the demo, as well as plugins and texture packs) but only gives you a state to play around in (you get the whole world in the full version of course). You also only get 15 minutes of gameplay (the timer pauses when you are in settings or anything, it's only when you are fling that it counts down) but once the 15 minutes is up you can just restart the app and play again.

Here is the demo: http://www.x-plane.com/downloads/x-plane_10_demo/

I highly recommend it (and your mac will easily run it, my iMac has the 750m GPU and I get around 45fps maxed out at 1080p with 4X AA and 4X AF with everything set to full apart from HDR mode).

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And I have used FSX for many years. Just trying the demo of X-Plane and getting used to it for an hour and I already knew that I was missing out on so much.