Matt
There is so much out there now it is hard to say. Whatever you do you will need a new Motherboard.
Personally I would recommend 2 Hard Drives, put FS9 on the C drive in it's normal place, and FSX on the other one by creating a folder Flightsim X or something and put it in there. Solid State Hard Drives seem to be the thing now, depends on price. I still have the normal ones, both are Velociraptors, which run faster than the others. 1 is 150 gigs with the Op. System, programs including FS9, and general stuf. The other is 75 gigs for FSX, FS Commander and Squawkbox . Don't go too big with drives, just what you think you will need. There is no point getting the huge drives they have now if you are only going to use one tenth of them.
Windows 7 X 64 has to be the Op. system so you can load it up with Ram and it will recognise all of it. (I have 8 gigs with room for another eight
. There are some concerns with W7 X 64, such as the often mentioned UACs (User Account Controls), which block programs accessing each other's information. There are several posts about that on these forums.
The best Display Adapter you can afford, I got the Nvidia GE Force GTX 460.
Remember it is important to have everything "balanced" and working together, rather than having one superfast component that is constantly slowed by other parts of the system which, will only work as fast as the slowest component.
That's about it.
Have fun
John