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.
See Tom's Gaming Graphics Charts Q3/2008 - http://www.tomshardware.com/charts/gaming-graphics-charts-q3-2008/benchmarks,30.html
I've been a Flight Sim enthusiast for 15 years, so I've built all my PC's over the years as dedicated FSX rigs.