Athlon Dual or old Athlon XP

HiI'm a returning simmer who's taken a couple of years out from simming, but with the advent of X have been tempted back into the fold.I find myself with two pcs, one and oldish Shuttle based on a Athlon XP 2600+ with a ATI 9600 and a new Athlon 64 3000+ with a Nvidia 6600 PCI express which runs my HTPC. both PC have only 500mb ram and run XP pro. both video card have 128mb ram on board.Anyhow I got hold of the X demo off the PC Pilot Mag and ran it up on both machines. On the 64 with a big plasma it ran like a dog, useless, waste of time.Just for a joke I ran it up on my old machine and was able to get around 18fps while island hopping.so the question is.Which machine should I base my FSX upgrades on.Option1 upgrade the older XP machine with more memory faster processor ( though I don't think they get much faster than 3000) maybe a newer video card.Option 2 upgrade the newer machine to an Athlon dual core, again more memory, maybe a graphics card.Common sense suggests upgrading the newer machine as it'll allow for running either the 64bit XP or Vista. On the other hand if FS X is not capable of taking advantage of Vista or dual core then the Athlon XP upgrades will be cheaper.For a real laugh I reinstalled FS9 on my XP machine. 30 fps off miegs field, wonderful!Seasons greeting.Ian