Here is 1 thread you could look at which may help in your selection:
http://www.aavirtual.com/forum/viewtopi ... 19&t=17426
And here is another one:
http://www.aavirtual.com/forum/viewtopi ... 32&t=15038
Now, I guess it depends on what Frame Rate you want or expect. I originally started with a dual core and GTX260 video card, I had to back off all of the sliders to get a reasonable frame rate of around 15 FPS but this would regularly drop down to around 5 FPS. I eventually gave up on FSX and went back to FS9.
I have upgraded recently to a pure gaming computer with the following specs:
Intel core i7 950/3GHZ/4.8T.
GA-X58A-UD5 mother board.
12GB RAM.
OCZ SSD 2.5" SATA 120GB drive (solid state drive).
Radeon HD6850 Video Card.
Creative X-fi XtremeAudio Card.
Windows 7 Pro 64 bit.
Phantom NZXT case.
Of course it also has a couple of other items installed but the above is the most important bits.
I now run FSX with all the sliders maxed out and have the frame rate set at 20 FPS, it occasionally drops to 18 or 19 but it is now a pleasure to fly.
You have to set up the "affinity mask" to have the FSX run in the 4 cores. I have found that even at 20 FPS the processor is still only running at around 50%.
I don't believe you can buy performance cheaply, in fact you probably need a purpose built computer for best results