I predict largely marketing-driven requirements specifications for FSX -- in particular for Vista. So if you look for whatever the high-end Vista specs are going to be, you can be sure that they'll be adequate for FSX at high settings.
I further predict that magic numbers such as >= 2.0 GHz AMD processors or >= 3.0 GHz Pentium IV processors, and perhaps 1.6-1.8 GHz Conroe processors should they be available. In part, this will be determined by what the manufacturers and resellers want to sell around the time. 64-bit capable for optimal marketing compatibility (YMMV regarding actual device driver support; buy a pre-built name-brand computer and don't add anything, and you should be fine.)
Now my note this has little to do with actual performance, and everything to do with marketing, selling new hardware, OS's, other software and learned cynicism.
What would I do? One of:
(1) If I or my end user cared about marketing compatibility, I'd hold off building a system or buy one that will be "Vista ready", whatever that means. Regarding video cards, the current higher-end DX9 SM3 cards are probably going to be fine, but greater marketing compatibility will be had when D3D 10 video cards and drivers are released.
(2) Alternatively, get the most cost-effective hardware from a performance standpoint. For now, that'd probably be an AMD processor, perhaps a dual core, perhaps an Opteron, and overclock it. Some consideration regarding dual core. Will FS X be optimized for dual cores? I predict that it will, eventually at least. But if the price is very sensitive, as it generally is for older people in my experience, I wouldn't worry and chop the core down to a single overclockable one (to get more value/$).
The 6600 GT fits into plan (2). It's going to be replaced by the 7600 series over time, but if you already have one, I think it'll be fine to pass it on.
I think the OP is already commited to something like plan (2), and I think it'll be just fine; there's a lot that just cannot be judged adequately until Vista / FSX are released and tested, and at that time, final tweeks or even massive upgrades can be done if needed/wanted, however I think that'd be unlikely from a need perspective (just turn down some of the settings); wants are another matter.