I'd personally take a guess that it would run just about adequately on the lowest settings (don't hold me to that, it's a guess, and if not, you'll have bought a 30-quid drinks coaster), although it would probably need a low screen resolution and all the graphics whistles and bells turned off*
* this is also providing your graphics card supports the pixel shader requirements - which are not something that affects FSX, being that FS still basically uses the same old graphics engine of earlier FS versions and doesn't require a pixel-shader 2.0-capable graphics card.
If your graphics card cannot support pixel shader 2.0, forget it, it just isn't going to run the 3D.
I can't imagine what a pain in the ass it must be waiting for FSX to load up on 512 meg of RAM though, you must have the patience of a saint
You might find that if SH4 does run on your rig, it will take a long time to load, and it will probably be very choppy on accelerated time, as even machines with a lot of RAM are having issues with that.
I'd seriously consider sticking some more RAM in