I have to agree with Jim. The GT 520 is more suited to a word processor/browser.
A GTX 550 Ti or HD 7750 will be pretty good, on a budget. GTX 560 Ti or HD 7850 will be better, if the price is not a major detractor.
The nVidia offerings historically look better in FSX, but, your wallet may help decide.
FSX hardware demands are not comparable with FS9 either. Not by a long shot.
FSX is so demanding, that a rig that is capable of marginally maxing out everything on FS9 getting 25-30FPS, will be lucky to get 20FPS with med/high settings in FSX. How do I know? That perfectly describes my old computer!
My purpose built new rig does do extremely well, but, still nowhere near max. A bummer after putting $50/month aside for almost 6 years!
I don't expect a computer that can max FSX out, and get 30FPS everywhere, to be generally available for a few years yet...Don