So lets see, your running that Rig + Vista, and you want to run FSX? Forgive me while I laugh. Seriously you could throw in another gig of RAM, and then get a 7800GS, but FSX still isn't going to preform the way you want it to.
My best recomendation would be to just start over. Build a whole new rig. Its going to be a much better upgrade than trying to nickel and dime this one into preforming.
Cheers
Cameron
Or the alternative is and im guessing socket 754 is Socker A AMD?
new mobo, another gig of ram and a new CPU, which if u where to get a AMD s939 x2 would not be to expensive (considering) but an intel would cost you more for what would be recomended on these forums. (e6600 and for GPU evga 8800GTX)
however
replacing the mobo, cpu and gpu is basicly the cost of a new rig and the basis so you would be looking at basicly a new rig, to recomend updates a MAX price would be needed.
and if u just want to upgrade a price is also needed
if u want to upgrade
on ebay u can get some socket A processor's from laptops which run cooler and are very overclockable, i know as a friend of mine has done just this and currebnty has a 2ghz barton at 2.4ghz

which is a more than acceptable speed for 2days budget PC
for GPU either a radeon x1900x AGP card or the nvidia 7600 AGP card
i dnt know how many mem slots u have, if u have 2, try and get 2 match sticks of 1gb or 512 ( if 2 slots stick 512md, wht u have atm), i would recomend spending thta little bit more and getting the 2 1gb sticks if ur mobo can take it

that is what i would recomend for the upgrade, CTjoyce is recomending the top of the range (which is fine if u can afford) but for budget PC my recomendations are not that bad

but the decision is up 2 u
john