A faster HDD (or SSD for that matter) will not increase the framerate of a game by itself. However, there are several different kinds of poor performance you can have, and a faster drive will help in a lot of these situations.
FSX uses data streaming to provide the continuous world without pausing for loading, this means the initial loading screen is only loading the scenery and other data relevant for your initial position, so it will continue to load as it goes... If your HDD is fragged or has too much stuff on it, (not to mention the added lag from windows and it's swap file), this streaming will be slow and you will have stutters because the HDD will be bottlenecking the whole rig.
A faster drive can open up that bottleneck and will give you not faster framerates, but smoother performance overall.
It's definitely worth it IMO, if I had the money at hand I'd buy one right now
Cheers
Pages: 
