Fsx isnt very demanding when it comes to response times.
I would look at video play back and picture quality as more importat issues.
Go to Best Buy or somewhere similar and look at differing monitors. Take along a video for something to compare with.
You dont have to buy it from there if you find a good deal somewhere else but it gives you a chance to see many different models side by side.
A good time to do it is like Tues. morning when no one is there and you can usually find someone who will spend time with you to make any changes you may want.