Bit of advice that might help others to get some improvements in their frame rates with FSX........
I installed the nHancer program together with some other adjustments, but ended up with much lower frame rates. It took some time to find out that the reason was that the box for dual core use was not checked in the options boxes of Nhancer (it is on the third menu and seems to be single core processor by default). So the advice is to check the settings if you have installed nHancer to make sure that it is set up to use dual core processors.
Also if you are using Vista then check that the affinity is set to allow FSX to use dual core (or quad core if you are lucky enough to have it!!). Once you have FSX running, then start the task manager and right click on the FSX.exe setting and then set the affinity to dual core.
Hope that this helps!!
Smooth landings.