If the ONLY thing you use your dedicated system for is FS9,
then it would make little sense to move to W7, FS9 was designed to
run at it's best under XP.

If anything , you should upgrade to XPpro 64bit, but, this is ONLY if you do nothing else with it, because you won't need anything from an OS that is nearly a decade newer than the program you want to run.
If you're asking because of performance issues you might be having, then that can't be solved by switching to a new OS.

FS, any version, is hardware hungry, it could care less which OS is running .

Most all people who use FS9, are still running XP, because it's
getting old enough to run fast on today's 'bargin' systems, which was yesterday's top of the line.