Yay! I'm sorry I only saw this thread now!
I just recently bought X-Plane 9 and some add-ons for it. I bought Real-environments Extreme for it as well as a forest texture enhancement pack. What I like about X-Plane is that it seems to look better than FSX and also doesn't need a monster machine to run either. The downside is that the planes are not as detailed as FSX's, but a lot of critics have said that X-Plane 9 has more realistic flight modeling than FSX making for a more accurate "sim".
Here's a link to REX for X-Plane 9 to give you an idea:
http://www.realenvironmentxtreme.com/xplane.html
I'd also like to add that Lock-On Modern Air Combat is a complete waste of time. It was a promising title but it is FULL of bugs and the patches are VERY large in size, and surprisingly ended up making the game even worse. I had the title on release day and then promptly got rid of it a few months later when there was no improvement.
People should also refrain from mentioning any "action" title such as HAWX in a flight-simulator thread. It's nowhere NEAR a simulator and it's like calling Need for Speed a realistic driving game.
I find that flight simulator communities are also much more serious and go as far to say that flight simulators are not classified as "games" but rather a hobby and / or a tool for real world flying. X-Plane 9 for example comes in a professional version which can be used to train real-world pilots.