FSX is the most CPU bound gaming title available today. It demands as much CPU horsepower as you can afford to throw at it. FSX frame rate is ~ 85% dependent upon CPU Cores and overclocking, so an i7 @ 4.0 Ghz yields responsive frame rates.
Although FSX runs best on mainstream nVidia GPU's, it also runs best on a single GPU, rather than SLI / CF / X2 GPU's, since multiple GPU's divert valuable CPU interrupts, which has the effect of actually reducing frame rates.
I tried 3 years ago to buy a PC capable of running FSX well, unfortunately I made the fatal mistake of buying a pre-packaged computer from Dell, as I had not yet discovered self-building. It performed OK on my system, but I am now in the process of completely overhauling my entire PC. So far I have only upgraded my video card; to an ATI Radeon HD4850 with 1gig dedicated memory. I would highly recommend this card, it is reasonably priced and performs beautifully. By itself it increased my FSX experience 10 fold. I run the game graphics on High (with obvious tweaks to individual sliders) and consistently see FPS over 50.
32 bit OS
Processor: Pentium Core 2 Duo 2.13 g Hz
RAM: 2-gig DDR2
Video Card: ATI Radeon HD4850 1 gig dedicated
PSU: Corsair 750TX