It will improve load times a lot but not FPS or smoothness. I don't have a massive FSX install (80GB) but FSX loads pretty quick on my VR - 1min, 15 sec.
A SSD would probably be even better but they are really expensive - +$500 for a 256GB one. I would imagine that a SSD may improve smoothness and fps because they don't get fragmented like a VR.
BTW, a VelociRaptor is a high speed hard disk and not a SSD.
If budget isn't a major concern, get a 600GB VR. If you don't mind longer load times and you are watching your pennies, go for a 7200 RPM Drive. If money is no object, go for a 500GB SSD. What ever you do, put FSX on its own drive and not the drive with the OS.
In all, I have three hard drives:
1TB WD Caviar Black - Windows 7 and general stuff
150GB WD Raptor - games, ArcGIS
200GB WD VelociRaptor - FSX