One of the best things you can do to improve the performance of a resource-hungry application like Flight Simulator X is, obviously, to feed it more resources. It isn't always obvious, though, how many of those resources are eaten up by other programs, services, etc., that run in the background and that you may not need while using FSX.
Microsoft MVP Owen Hewitt has been trying out a new tool called AlacrityPC that is designed to shut down non-essential programs and services automatically before launching something like FSX, then restarting them after the fact. Owen said "After a few hours of tweaking this utility on my system, I'm seeing a tremendous improvement in performance of FSX."
Huge Disclaimer: These sorts of tools can be very, very useful, and this one looks as if it has lots of safeguards and things, but there are always risks of instability when you start shutting things down that you may not know much about.
With that out of the way, check out Owen's blog post here, and follow the links if you decide to have a look. If it works for you, let us know!