I've found that Flight Simulator X runs better on Vista than it does on XP when the graphics are cranked up. This was claimed to be the case by Microsoft when FSX was released and Vista still had not been released, and many dismissed the claim as nonsense, now that Vista is available, detractors are having to eat their words a bit. I've definitely noticed that FSX does seem to allow the autogen scenery to run on full throttle when you have it on a Vista system, and the autogen is one of the big framerate killers in XP. The other bottleneck for FSX is RAM, or the lack of it, too little RAM and too high a setting in FSX results in the sim giving up even trying to display textures on the terrain at full resolution, resulting in what most people refer to as 'the blurries'. So you need plenty of RAM (and by that, I mean 4GB or more), but there is a caveat here; Vista will allow you to configure a USB flash drive as supplementary RAM, so you can boost performance a little in that way with the newer OS if you have to.
I'm not a fan of Vista in a lot of ways, and I certainly don't like the looks and layout of it, but I can't deny that it does run some stuff better than XP.
So, personally, I'd recommend putting Vista (yes, I really did say that) and lots of RAM high on your spec list too, if you want FSX on full throttle graphically.
Incidentally, the cheapest version of Vista does not support some of these features by the way, so be careful which one you buy if you choose to get it.
Chock